French Republican calendar

Write a program to convert dates between the Gregorian calendar and the French Republican calendar.

French Republican calendar
You are encouraged to solve this task according to the task description, using any language you may know.

The year 1 of the Republican calendar began on 22 September 1792. There were twelve months (Vendémiaire, Brumaire, Frimaire, Nivôse, Pluviôse, Ventôse, Germinal, Floréal, Prairial, Messidor, Thermidor, and Fructidor) of 30 days each, followed by five intercalary days or Sansculottides (Fête de la vertu / Virtue Day, Fête du génie / Talent Day, Fête du travail / Labour Day, Fête de l'opinion / Opinion Day, and Fête des récompenses / Honours Day). In leap years (the years 3, 7, and 11) a sixth Sansculottide was added: Fête de la Révolution / Revolution Day.

As a minimum, your program should give correct results for dates in the range from 1 Vendémiaire 1 = 22 September 1792 to 10 Nivôse 14 = 31 December 1805 (the last day when the Republican calendar was officially in use). If you choose to accept later dates, be aware that there are several different methods (described on the Wikipedia page) about how to determine leap years after the year 14. You should indicate which method you are using. (Because of these different methods, correct programs may sometimes give different results for dates after 1805.)

Test your program by converting the following dates both from Gregorian to Republican and from Republican to Gregorian:

• 1 Vendémiaire 1 = 22 September 1792

• 1 Prairial 3 = 20 May 1795

• 27 Messidor 7 = 15 July 1799 (Rosetta Stone discovered)

• Fête de la Révolution 11 = 23 September 1803

• 10 Nivôse 14 = 31 December 1805

BBC BASIC

Computes leap years using the "continuous" method: a year in the Republican calendar is a leap year if and only if the number of the following year is divisible by 4 but not by 100 unless also by 400. No attempt is made to deal with ill-formed or invalid input dates. <lang bbcbasic>REM >frrepcal

DIM gregorian$(11) DIM gregorian%(11) DIM republican$(11) DIM sansculottides$(5) gregorian$() = "January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December" gregorian%() = 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 REM 7-bit ASCII encoding, so no accents on French words republican$() = "Vendemiaire", "Brumaire", "Frimaire", "Nivose", "Pluviose", "Ventose", "Germinal", "Floreal", "Prairial", "Messidor", "Thermidor", "Fructidor" sansculottides$() = "Fete de la vertu", "Fete du genie", "Fete du travail", "Fete de l'opinion", "Fete des recompenses", "Fete de la Revolution"

PRINT "*** French Republican ***" PRINT "*** calendar converter ***" PRINT "Enter a date to convert, in the format 'day month year'" PRINT "e.g.: 1 Prairial 3," PRINT " 20 May 1795." PRINT "For Sansculottides, use 'day year'" PRINT "e.g.: Fete de l'opinion 9." PRINT "Or just press 'RETURN' to exit the program." PRINT REPEAT

 INPUT LINE "> " src$
 IF src$ <> "" THEN
   PROC_split(src$, day%, month%, year%)
   REM for simplicity, we assume that years up to 1791 are Republican
   REM and years from 1792 onwards are Gregorian
   IF year% < 1792 THEN
     REM convert Republican date to number of days elapsed
     REM since 21 September 1792, then convert that number
     REM to the Gregorian date
     PROC_day_to_gre(FN_rep_to_day(day%, month%, year%), day%, month%, year%)
     PRINT; day%; " "; gregorian$(month% - 1); " " year%
   ELSE
     REM convert Gregorian date to Republican, via
     REM number of days elapsed since 21 September 1792
     PROC_day_to_rep(FN_gre_to_day(day%, month%, year%), day%, month%, year%)
     IF month% = 13 THEN
       PRINT sansculottides$(day% - 1); " "; year%
     ELSE
       PRINT; day%; " "; republican$(month% - 1); " "; year%
     ENDIF
   ENDIF
 ENDIF

UNTIL src$ = "" END

DEF PROC_split(s$, RETURN d%, RETURN m%, RETURN y%) LOCAL month_and_year$, month$, months$(), i% DIM months$(11) IF LEFT$(s$, 4) = "Fete" THEN

 m% = 13
 FOR i% = 0 TO 5
   IF LEFT$(s$, LEN sansculottides$(i%)) = sansculottides$(i%) THEN
     d% = i% + 1
     y% = VAL(RIGHT$(s$, LEN s$ - LEN sansculottides$(i%) - 1))
   ENDIF
 NEXT

ELSE

 d% = VAL(LEFT$(s$, INSTR(s$, " ") - 1))
 month_and_year$ = MID$(s$, INSTR(s$, " ") + 1)
 month$ = LEFT$(month_and_year$, INSTR(month_and_year$, " ") - 1)
 y% = VAL(MID$(month_and_year$, INSTR(month_and_year$, " ") + 1))
 IF y% < 1792 THEN months$() = republican$() ELSE months$() = gregorian$()
 FOR i% = 0 TO 11
   IF months$(i%) = month$ THEN m% = i% + 1
 NEXT

ENDIF ENDPROC

DEF FN_gre_to_day(d%, m%, y%) REM modified & repurposed from code given at REM https://www.staff.science.uu.nl/~gent0113/calendar/isocalendar_text5.htm IF m% < 3 THEN

 y% -= 1
 m% += 12

ENDIF = INT(365.25 * y%) - INT(y% / 100) + INT(y% / 400) + INT(30.6 * (m% + 1)) + d% - 654842

DEF FN_rep_to_day(d%, m%, y%) REM assume that a year is a leap year iff the _following_ year is REM divisible by 4, but not by 100 unless also by 400 REM REM other methods for computing Republican leap years exist IF m% = 13 THEN

 m% -= 1
 d% += 30

ENDIF IF FN_rep_leap(y%) THEN d% -= 1 = 365 * y% + INT((y% + 1) / 4) - INT((y% + 1) / 100) + INT((y% + 1) / 400) + 30 * m% + d% - 395

DEF PROC_day_to_gre(day%, RETURN d%, RETURN m%, RETURN y%) y% = INT(day% / 365.25) d% = day% - INT(365.25 * y%) + 21 y% += 1792 d% += INT(y% / 100) - INT(y% / 400) - 13 m% = 8 WHILE d% > gregorian%(m%)

 d% -= gregorian%(m%)
 m% += 1
 IF m% = 12 THEN
   m% = 0
   y% += 1
   IF FN_gre_leap(y%) THEN gregorian%(1) = 29 ELSE gregorian%(1) = 28
 ENDIF

ENDWHILE m% += 1 ENDPROC

DEF PROC_day_to_rep(day%, RETURN d%, RETURN m%, RETURN y%) LOCAL sansculottides% y% = INT(day% / 365.25) IF FN_rep_leap(y%) THEN y% -= 1 d% = day% - INT(365.25 * y%) + INT((y% + 1) / 100) - INT((y% + 1) / 400) y% += 1 m% = 1 IF FN_rep_leap(y%) THEN sansculottides% = 6 ELSE sansculottides% = 5 WHILE d% > 30

 d% -= 30
 m% += 1
 IF m% = 13 THEN
   IF d% > sansculottides% THEN
     d% -= sansculottides%
     m% = 1
     y% += 1
     IF FN_rep_leap(y%) THEN sansculottides% = 6 ELSE sansculottides% = 5
   ENDIF
 ENDIF

ENDWHILE ENDPROC

DEF FN_rep_leap(year%) REM see comment at the beginning of FN_rep_to_day = ((year% + 1) MOD 4 = 0 AND ((year% + 1) MOD 100 <> 0 OR (year% + 1) MOD 400 = 0))

DEF FN_gre_leap(year%) = (year% MOD 4 = 0 AND (year% MOD 100 <> 0 OR year% MOD 400 = 0))</lang> Output for the test dates:

*** French  Republican ***
*** calendar converter ***
Enter a date to convert, in the format 'day month year'
e.g.: 1 Prairial 3,
      20 May 1795.
For Sansculottides, use 'day year'
e.g.: Fete de l'opinion 9.
Or just press 'RETURN' to exit the program.

> 1 Vendemiaire 1
22 September 1792
> 22 September 1792
1 Vendemiaire 1
> 1 Prairial 3
20 May 1795
> 20 May 1795
1 Prairial 3
> 27 Messidor 7
15 July 1799
> 15 July 1799
27 Messidor 7
> Fete de la Revolution 11
23 September 1803
> 23 September 1803
Fete de la Revolution 11
> 10 Nivose 14
31 December 1805
> 31 December 1805
10 Nivose 14

Output for a few subsequent dates:

> 18 March 1871
27 Ventose 79
> 25 August 1944
7 Fructidor 152
> 19 September 2016
Fete du travail 224

FreeBASIC

Translation of: BBC BASIC

Computes leap years using the "continuous" method: a year in the Republican calendar is a leap year if and only if the number of the following year is divisible by 4 but not by 100 unless also by 400. No attempt is made to deal with ill-formed or invalid input dates. <lang freebasic>' version 18 Pluviose 227 ' compile with: fbc -s console ' retained the original comments for then BBC BASIC entry

1. Macro rep_leap (_year)

   ' see comment at the beginning of rep_to_day
   ((_year +1) Mod 4 = 0 And ((_year +1) Mod 100 <> 0 Or (_year +1) Mod 400 = 0))

1. EndMacro

1. Macro gre_leap (_year)

   (_year Mod 4 = 0 And (_year Mod 100 <> 0 Or _year Mod 400 = 0))

1. EndMacro

Dim Shared As UInteger gregorian(11) => {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31} Dim Shared As String gregorian_s(11), republican(11), sanscolottides(5) ' 7-bit ASCII encoding, so no accents on French words Data "January", "February", "March", "April", "May", "June" Data "July", "August", "September", "October", "November", "December" Data "Vendemiaire", "Brumaire", "Frimaire","Nivose", "Pluviose", "Ventose" Data "Germinal", "Floreal", "Prairial", "Messidor", "Thermidor", "Fructidor" Data "Fete de la Vertu", "Fete du Genie", "Fete du Travail", "Fete de l'Opinion" Data "Fete des Recompenses","Fete de la Revolution" Restore For i As UInteger = 0 To 11

   Read gregorian_s(i)

Next For i As UInteger = 0 To 11

   Read republican(i)

Next For i As UInteger = 0 To 5

   Read sanscolottides(i)

Next

Sub split(s As String, ByRef d As UInteger, ByRef m As UInteger, ByRef y As UInteger)

   Dim As String month_and_year, Month
   Dim As UInteger i

   s = LCase(Trim(s)) : d = 0 : m = 0 : y = 0
   If Left(s,4) = "fete" Then
       m = 13
       For i = 0 To 5
           If Left(s, Len(sanscolottides(i))) = LCase(sanscolottides(i)) Then
               d = i +1
               y = Val(Right(s, Len(s) - Len(sanscolottides(i)) -1))
           End If
       Next
   Else
       d = Val(Left(s, InStr(s, " ") -1))
       month_and_year = Mid(s, InStr(s, " ") +1)
       Month = Left(month_and_year, InStr(month_and_year, " ") -1)
       y = Val(Mid(month_and_year, InStr(month_and_year, " ") +1))
       If y < 1792 Then
           For i = 0 To 11
               If LCase(republican(i)) = Month Then m = i +1
           Next
       Else
           For i = 0 To 11
               If LCase(gregorian_s(i)) = Month Then m = i +1
           Next
       End If
   End If

End Sub

Sub day_to_gre(Day As UInteger, ByRef d As UInteger, ByRef m As UInteger, ByRef y As UInteger)

   y = Fix(Day / 365.25)
   d = Day - Fix(365.25 * y) + 21
   y += 1792
   d += y \ 100 - y \ 400 - 13
   m = 8

   While d > gregorian(m)
       d -= gregorian(m)
       m += 1
       If m = 12 Then
           m = 0
           y += 1
           If gre_leap(y) Then gregorian(1) = 29 Else gregorian(1) = 28
       End If
   Wend
   gregorian(1) = 28
   m += 1

End Sub

Function gre_to_day(d As UInteger, m As UInteger, y As UInteger) As UInteger

   ' modified & repurposed from code given at
   ' https://www.staff.science.uu.nl/~gent0113/calendar/isocalendar_text5.htm

   If m < 3 Then
       y -= 1
       m += 12
   End If
   Return Fix(365.25 * y) - y \ 100 + y \ 400 + Fix(30.6 * (m +1)) + d - 654842

End Function

Function rep_to_day(d As UInteger, m As UInteger, y As UInteger) As UInteger

   ' assume that a year is a leap year iff the _following_ year is
   ' divisible by 4, but not by 100 unless also by 400
   '
   ' other methods for computing republican leap years exist

   If m = 13 Then
       m -= 1
       d += 30
   End If
   If rep_leap(y) Then d -= 1

   Return 365 * y + (y +1) \ 4 - (y +1) \ 100 + (y +1) \ 400 + 30 * m + d - 395

End Function

Sub day_to_rep(Day As UInteger, ByRef d As UInteger, ByRef m As UInteger, ByRef y As UInteger)

   Dim As UInteger sansculottides = 5

   y = Fix(Day / 365.25)
   If rep_leap(y) Then y -= 1
   d = Day - Fix(365.25 * y) + (y +1) \ 100 - (y +1) \ 400
   y += 1
   m = 1
   If rep_leap(y) Then sansculottides = 6
   While d > 30
       d -= 30
       m += 1
       If m = 13 Then
           If d > sansculottides Then
               d -= sansculottides
               m = 1
               y += 1
               If rep_leap(y) Then sansculottides = 6 Else sansculottides = 5
           End If
       End If
   Wend

End Sub

' ------=< main >=------

Dim As UInteger Day, Month, Year Dim As String src

Print "*** French Republican ***" Print "*** calendar converter ***" Print "Enter a date to convert, in the format 'day month year'" Print "e.g.: 1 Prairial 3," Print " 20 May 1795." Print "For Sansculottides, use 'day year'" Print "e.g.: Fete de l'opinion 9." Print "Or just press 'RETURN' to exit the program." Print

Do

   Line Input "> ", src
   If src <> "" Then
       split(src, Day, Month, Year)
       If Day = 0 Or Month = 0 Or Year <= 0 Then
           Print "Error in input"
           Continue Do
       End If
       ' for simplicity, we assume that years up to 1791 are republican
       ' and years from 1792 onwards are gregorian
       If Year < 1792 Then
           ' convert republican date to number of days elapsed
           ' since 21 september 1792, then convert that number
           ' to the gregorian date
           day_to_gre(rep_to_day(Day, Month, Year),Day, Month, Year)
           Print; Day; " "; gregorian_s(Month -1); " "; Year
       Else
           ' convert gregorian date to republican, via
           ' number of days elapsed since 21 september 1792
           day_to_rep(gre_to_day(Day, Month, Year), Day, Month, Year)
           If Month = 13 Then
               Print sanscolottides(Day -1); " "; Year
           Else
               Print ; Day; " "; republican(Month -1); " "; Year
           End If
       End If
   End If

Loop Until src = ""

End</lang>

Output:

> 1 Vendemiaire 1                   > 22 September 1792          > 1 Vendemiaire 1          > 23 September 1806          > 1 Vendemiaire 15
22 September 1792                   1 Vendemiaire 1              22 September 1792          1 Vendemiaire 15             23 September 1806
> 22 September 1792                 > 22 September 1793          > 1 Vendemiaire 2          > 24 September 1807          > 1 Vendemiaire 16
1 Vendemiaire 1                     1 Vendemiaire 2              22 September 1793          1 Vendemiaire 16             24 September 1807
> 1 Prairial 3                      > 22 September 1794          > 1 Vendemiaire 3          > 23 September 1808          > 1 Vendemiaire 17
20 May 1795                         1 Vendemiaire 3              22 September 1794          1 Vendemiaire 17             23 September 1808
> 20 May 1795                       > 23 September 1795          > 1 Vendemiaire 4          > 23 September 1809          > 1 Vendemiaire 18
1 Prairial 3                        1 Vendemiaire 4              23 September 1795          1 Vendemiaire 18             23 September 1809
> 27 Messidor 7                     > 22 September 1796          > 1 Vendemiaire 5          > 23 September 1810          > 1 Vendemiaire 19
15 July 1799                        1 Vendemiaire 5              22 September 1796          1 Vendemiaire 19             23 September 1810
> 15 July 1799                      > 22 September 1797          > 1 Vendemiaire 6          > 24 September 1811          > 1 Vendemiaire 20
27 Messidor 7                       1 Vendemiaire 6              22 September 1797          1 Vendemiaire 20             24 September 1811
> Fete de la Revolution 11          > 22 September 1798          > 1 Vendemiaire 7          > 23 September 2015          > 1 Vendemiaire 224
23 September 1803                   1 Vendemiaire 7              22 September 1798          1 Vendemiaire 224            23 September 2015
> 23 September 1803                 > 23 September 1799          > 1 Vendemiaire 8          > 22 September 2016          > 1 Vendemiaire 225
Fete de la Revolution 11            1 Vendemiaire 8              23 September 1799          1 Vendemiaire 225            22 September 2016
> 10 Nivose 14                      > 23 September 1800          > 1 Vendemiaire 9          > 22 September 2017          > 1 Vendemiaire 226
31 December 1805                    1 Vendemiaire 9              23 September 1800          1 Vendemiaire 226            22 September 2017
> 31 December 1805                  > 23 September 1801          > 1 Vendemiaire 10         > 22 September 2018          > 1 Vendemiaire 227
10 Nivose 14                        1 Vendemiaire 10             23 September 1801          1 Vendemiaire 227            22 September 2018
                                    > 23 September 1802          > 1 Vendemiaire 11         > 23 September 2019          > 1 Vendemiaire 228
                                    1 Vendemiaire 11             23 September 1802          1 Vendemiaire 228            23 September 2019
                                    > 24 September 1803          > 1 Vendemiaire 12         > 22 September 2020          > 1 Vendemiaire 229
                                    1 Vendemiaire 12             24 September 1803          1 Vendemiaire 229            22 September 2020
                                    > 23 September 1804          > 1 Vendemiaire 13
                                    1 Vendemiaire 13             23 September 1804
                                    > 23 September 1805          > 1 Vendemiaire 14
                                    1 Vendemiaire 14             23 September 1805

F#

<lang fsharp> // French Republican Calander: Nigel Galloway. April 16th., 2021 let firstDay=System.DateTime.Parse("22/9/1792") type monthsFRC= Vendemiaire = 0

              |Brumaire    =  30
              |Frimaire    =  60
              |Nivose      =  90
              |Pluviose    = 120
              |Ventose     = 150
              |Germinal    = 180
              |Floral      = 210
              |Prairial    = 240
              |Messidor    = 270
              |Thermidor   = 300
              |Fructidor   = 330
              |Virtue      = 360
              |Talent      = 361
              |Labour      = 362
              |Opinion     = 363
              |Honours     = 364
              |Revolution  = 365

type months= January = 1

           |February   =  2
           |March      =  3
           |April      =  4
           |May        =  5
           |June       =  6
           |July       =  7
           |August     =  8 
           |September  =  9
           |October    = 10
           |November   = 11
           |December   = 12

let frc2Greg n (g:monthsFRC) l=firstDay+System.TimeSpan.FromDays(float((l-1)*365+l/4+(int g)+n-1)) let rec fG n g=let i=match g with 3 |7 |11->366 |_->365 in if n(1,enum<monthsFRC>(360+n),g) |(n,l)->(l+1,enum<monthsFRC>(n*30),g) let n=(frc2Greg 1 monthsFRC.Vendemiaire 1) in printfn "%d %s %d -> %d %A %d" 1 "Vendemiaire" 1 n.Day (enum<months> n.Month) n.Year let n=(frc2Greg 27 monthsFRC.Messidor 7) in printfn "%d %s %d -> %d %A %d" 27 "Messidor" 7 n.Day (enum<months> n.Month) n.Year let n=(frc2Greg 1 monthsFRC.Revolution 11) in printfn "%d %s %d -> %d %A %d" 1 "Revolution" 11 n.Day (enum<months> n.Month) n.Year let n=(frc2Greg 10 monthsFRC.Nivose 14) in printfn "%d %s %d -> %d %A %d" 10 "Nivose" 14 n.Day (enum<months> n.Month) n.Year let n,g,l=Greg2FRC(System.DateTime(1792,9,22)) in printfn "%d %s %d -> %d %A %d" 22 "September" 1792 n g l let n,g,l=Greg2FRC(System.DateTime(1799,7,15)) in printfn "%d %s %d -> %d %A %d" 15 "July" 1799 n g l let n,g,l=Greg2FRC(System.DateTime(1803,9,23)) in printfn "%d %s %d -> %d %A %d" 23 "September" 1803 n g l let n,g,l=Greg2FRC(System.DateTime(1805,12,31)) in printfn "%d %s %d -> %d %A %d" 31 "December" 1805 n g l </lang>

Output:

1 Vendemiaire 1 -> 22 September 1792
27 Messidor 7 -> 15 July 1799
1 Revolution 11 -> 23 September 1803
10 Nivose 14 -> 31 December 1805
22 September 1792 -> 1 Vendemiaire 1
15 July 1799 -> 27 Messidor 7
23 September 1803 -> 1 Revolution 11
31 December 1805 -> 10 Nivose 14

Go

Translation of: BBC Basic

A rather literal port, just for reference. Far from idiomatic Go. <lang go>package main

import (

   "bufio"
   "fmt"
   "os"
   "strconv"
   "strings"

)

var (

   gregorianStr = []string{"January", "February", "March",
       "April", "May", "June",
       "July", "August", "September",
       "October", "November", "December"}
   gregorian     = []int{31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31}
   republicanStr = []string{"Vendemiaire", "Brumaire", "Frimaire",
       "Nivose", "Pluviose", "Ventose",
       "Germinal", "Floreal", "Prairial",
       "Messidor", "Thermidor", "Fructidor"}
   sansculottidesStr = []string{"Fete de la vertu", "Fete du genie",
       "Fete du travail", "Fete de l'opinion",
       "Fete des recompenses", "Fete de la Revolution"}

)

func main() {

   fmt.Println("*** French  Republican ***")
   fmt.Println("*** calendar converter ***")
   fmt.Println("Enter a date to convert, in the format 'day month year'")
   fmt.Println("e.g.: 1 Prairial 3,")
   fmt.Println("      20 May 1795.")
   fmt.Println("For Sansculottides, use 'day year'")
   fmt.Println("e.g.: Fete de l'opinion 9.")
   fmt.Println("Or just press 'RETURN' to exit the program.")
   fmt.Println()
   for sc := bufio.NewScanner(os.Stdin); ; {
       fmt.Print("> ")
       sc.Scan()
       src := sc.Text()
       if src == "" {
           return
       }
       day, month, year := split(src)
       if year < 1792 {
           day, month, year = dayToGre(repToDay(day, month, year))
           fmt.Println(day, gregorianStr[month-1], year)
       } else {
           day, month, year := dayToRep(greToDay(day, month, year))
           if month == 13 {
               fmt.Println(sansculottidesStr[day-1], year)
           } else {
               fmt.Println(day, republicanStr[month-1], year)
           }
       }
   }

}

func split(s string) (d, m, y int) {

   if strings.HasPrefix(s, "Fete") {
       m = 13
       for i, sc := range sansculottidesStr {
           if strings.HasPrefix(s, sc) {
               d = i + 1
               y, _ = strconv.Atoi(s[len(sc)+1:])
           }
       }
   } else {
       d, _ = strconv.Atoi(s[:strings.Index(s, " ")])
       my := s[strings.Index(s, " ")+1:]
       mStr := my[:strings.Index(my, " ")]
       y, _ = strconv.Atoi(my[strings.Index(my, " ")+1:])
       months := gregorianStr
       if y < 1792 {
           months = republicanStr
       }
       for i, mn := range months {
           if mn == mStr {
               m = i + 1
           }
       }
   }
   return

}

func greToDay(d, m, y int) int {

   if m < 3 {
       y--
       m += 12
   }
   return y*36525/100 - y/100 + y/400 + 306*(m+1)/10 + d - 654842

}

func repToDay(d, m, y int) int {

   if m == 13 {
       m--
       d += 30
   }
   if repLeap(y) {
       d--
   }
   return 365*y + (y+1)/4 - (y+1)/100 + (y+1)/400 + 30*m + d - 395

}

func dayToGre(day int) (d, m, y int) {

   y = day * 100 / 36525
   d = day - y*36525/100 + 21
   y += 1792
   d += y/100 - y/400 - 13
   m = 8
   for d > gregorian[m] {
       d -= gregorian[m]
       m++
       if m == 12 {
           m = 0
           y++
           if greLeap(y) {
               gregorian[1] = 29
           } else {
               gregorian[1] = 28
           }
       }
   }
   m++
   return

}

func dayToRep(day int) (d, m, y int) {

   y = (day-1) * 100 / 36525
   if repLeap(y) {
       y--
   }
   d = day - (y+1)*36525/100 + 365 + (y+1)/100 - (y+1)/400
   y++
   m = 1
   sansculottides := 5
   if repLeap(y) {
       sansculottides = 6
   }
   for d > 30 {
       d -= 30
       m += 1
       if m == 13 {
           if d > sansculottides {
               d -= sansculottides
               m = 1
               y++
               sansculottides = 5
               if repLeap(y) {
                   sansculottides = 6
               }
           }
       }
   }
   return

}

func repLeap(year int) bool {

   return (year+1)%4 == 0 && ((year+1)%100 != 0 || (year+1)%400 == 0)

}

func greLeap(year int) bool {

   return year%4 == 0 && (year%100 != 0 || year%400 == 0)

}</lang>

Output:

$ go run frc.go
*** French  Republican ***
*** calendar converter ***
Enter a date to convert, in the format 'day month year'
e.g.: 1 Prairial 3,
      20 May 1795.
For Sansculottides, use 'day year'
e.g.: Fete de l'opinion 9.
Or just press 'RETURN' to exit the program.

> 1 Vendemiaire 1
22 September 1792
> 22 September 1792
1 Vendemiaire 1
> 1 Prairial 3
20 May 1795
> 20 May 1795
1 Prairial 3
> 27 Messidor 7
15 July 1799
> 15 July 1799
27 Messidor 7
> Fete de la Revolution 11
23 September 1803
> 23 September 1803
Fete de la Revolution 11
> 10 Nivose 14
31 December 1805
> 31 December 1805
10 Nivose 14
> 
$ 

More idiomatic

A start anyway. Computations extracted to a package, a type defined for French Republican Dates, time package from standard library used. Ignores invalid input rather than panicking. <lang go>package frc

import (

   "fmt"
   "strconv"
   "strings"
   "time"

)

type Date struct {

   Year  int
   Month int
   Day   int

}

func (d Date) String() string {

   if d.Month == 13 {
       return fmt.Sprintf("%s %d", sansculotidesStr[d.Day-1], d.Year)
   }
   return fmt.Sprintf("%d %s %d", d.Day, republicanStr[d.Month-1], d.Year)

}

func Parse(s string) (dt Date, ok bool) {

   f := strings.Fields(s)
   var err error
   switch {
   case len(f) == 3:
       if dt.Day, err = strconv.Atoi(f[0]); err != nil {
           return
       }
       i := 0
       for republicanStr[i] != f[1] {
           i++
           if i == len(republicanStr) {
               return
           }
       }
       dt.Month = i + 1
       if dt.Year, err = strconv.Atoi(f[2]); err != nil {
           return
       }
       ok = true
       return
   case len(f) > 3:
       for i, sc := range sansculotidesStr {
           if strings.HasPrefix(s, sc) {
               dt.Month = 13
               dt.Day = i + 1
               dt.Year, err = strconv.Atoi(s[len(sc)+1:])
               if err == nil {
                   ok = true
               }
               return
           }
       }
   }
   return

}

func (dt Date) ToGregorian() (y int, m time.Month, d int) {

   yr, mn, dy := dayToGre(repToDay(dt.Day, dt.Month, dt.Year))
   return yr, time.Month(mn), dy

}

func FromGregorian(y int, m time.Month, d int) Date {

   day, month, year := dayToRep(greToDay(d, int(m), y))
   return Date{Year: year, Month: month, Day: day}

} var (

   gregorian     = []int{31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31}
   republicanStr = []string{"Vendemiaire", "Brumaire", "Frimaire",
       "Nivose", "Pluviose", "Ventose",
       "Germinal", "Floreal", "Prairial",
       "Messidor", "Thermidor", "Fructidor"}
   sansculotidesStr = []string{"Fete de la vertu", "Fete du genie",
       "Fete du travail", "Fete de l'opinion",
       "Fete des recompenses", "Fete de la Revolution"}

)

func greToDay(d, m, y int) int {

   if m < 3 {
       y--
       m += 12
   }
   return y*36525/100 - y/100 + y/400 + 306*(m+1)/10 + d - 654842

}

func repToDay(d, m, y int) int {

   if m == 13 {
       m--
       d += 30
   }
   if repLeap(y) {
       d--
   }
   return 365*y + (y+1)/4 - (y+1)/100 + (y+1)/400 + 30*m + d - 395

}

func dayToGre(day int) (d, m, y int) {

   y = day * 100 / 36525
   d = day - y*36525/100 + 21
   y += 1792
   d += y/100 - y/400 - 13
   m = 8
   for d > gregorian[m] {
       d -= gregorian[m]
       m++
       if m == 12 {
           m = 0
           y++
           if greLeap(y) {
               gregorian[1] = 29
           } else {
               gregorian[1] = 28
           }
       }
   }
   m++
   return

}

func dayToRep(day int) (d, m, y int) {

   y = (day-1) * 100 / 36525
   if repLeap(y) {
       y--
   }
   d = day - (y+1)*36525/100 + 365 + (y+1)/100 - (y+1)/400
   y++
   m = 1
   sansculottides := 5
   if repLeap(y) {
       sansculottides = 6
   }
   for d > 30 {
       d -= 30
       m += 1
       if m == 13 {
           if d > sansculottides {
               d -= sansculottides
               m = 1
               y++
               sansculottides = 5
               if repLeap(y) {
                   sansculottides = 6
               }
           }
       }
   }
   return

}

func repLeap(year int) bool {

   return (year+1)%4 == 0 && ((year+1)%100 != 0 || (year+1)%400 == 0)

}

func greLeap(year int) bool {

   return year%4 == 0 && (year%100 != 0 || year%400 == 0)

}</lang> <lang go>package main

import (

   "bufio"
   "fmt"
   "os"
   "time"

   "frc"

)

func main() {

   fmt.Println("*** French  Republican ***")
   fmt.Println("*** calendar converter ***")
   fmt.Println("Enter a date to convert, in the format 'day month year'")
   fmt.Println("e.g.: 1 Prairial 3,")
   fmt.Println("      20 May 1795.")
   fmt.Println("For Sansculottides, use 'day year'")
   fmt.Println("e.g.: Fete de l'opinion 9.")
   fmt.Println("Or just press 'RETURN' to exit the program.")
   fmt.Println()
   for sc := bufio.NewScanner(os.Stdin); ; {
       fmt.Print("> ")
       sc.Scan()
       src := sc.Text()
       if src == "" {
           return
       }
       f, ok := frc.Parse(src)
       if ok {
           fmt.Println(f.ToGregorian())
           continue
       }
       t, err := time.Parse(`2 January 2006`, src)
       if err == nil {
           fmt.Println(frc.FromGregorian(t.Date()))
       }
   }

}</lang>

Julia

To stay within historical dates, will throw a domain error if the French Republican date is outside the interval the calendar was actually used. <lang julia>using Dates

const GC_FORMAT = DateFormat("d U y")

const RC_FIRST_DAY = Date(1792, 9, 22)

const MAX_RC_DATE = Date(1805, 12, 31)

const RC_MONTHS = [

   "Vendémiaire", "Brumaire", "Frimaire", "Nivôse", "Pluviôse", "Ventôse",
   "Germinal", "Floréal", "Prairial", "Messidor", "Thermidor", "Fructidor"

]

const RC_DAYS_IN_MONTH = 30

const RC_SANSCULOTTIDES = [

   "Fête de la vertu", "Fête du génie", "Fête du travail",
   "Fête de l'opinion", "Fête des récompenses", "Fête de la Révolution"

]

additionaldaysforyear(yr) = yr > 11 ? 3 : yr > 7 ? 2 : yr > 3 ? 1 : 0 additionaldaysformonth(mo) = 30 * (mo - 1) daysforFete(s) = findfirst(x -> x == s, RC_SANSCULOTTIDES) + 359

function togregorian(rc::String)

   yearstring, firstpart = reverse.(split(reverse(strip(rc)), r"\s+", limit=2))
   rcyear = parse(Int, yearstring)
   pastyeardays = (rcyear - 1) * 365 + additionaldaysforyear(rcyear)
   if isnumeric(firstpart[1])
       daystring, monthstring = split(firstpart, r"\s+", limit=2)
       nmonth = findfirst(x -> x == monthstring, RC_MONTHS)
       pastmonthdays = 30 * (nmonth - 1)
       furtherdays = parse(Int, daystring) + pastmonthdays + pastyeardays - 1
   else
       furtherdays = daysforFete(firstpart) + pastyeardays
   end
   gregorian = RC_FIRST_DAY + Day(furtherdays)
   if furtherdays < 0 || gregorian > MAX_RC_DATE
       throw(DomainError("French Republican Calendar date out of range"))
   end
   return Day(gregorian).value, monthname(Month(gregorian).value), Year(gregorian).value

end

function torepublican(gc::String)

   date = Date(DateTime(gc, GC_FORMAT))
   if date < RC_FIRST_DAY || date > MAX_RC_DATE
       throw(DomainError("French Republican Calendar date out of range"))
   end
   rcyear, rcdays = divrem(((date - RC_FIRST_DAY).value + 366), 365)
   rcdays -= additionaldaysforyear(rcyear)
   if rcdays < 1
       rcyear -= 1
       rcdays += 366
   end
   if rcdays < 361 
       nmonth, rcday = divrem(rcdays, 30)
       return rcday, RC_MONTHS[nmonth + 1], rcyear
   else
       return RC_SANSCULOTTIDES[rcdays - 360], rcyear
   end

end

const republican = [

   "1 Vendémiaire 1", "1 Prairial 3", "27 Messidor 7",
   "Fête de la Révolution 11", "10 Nivôse 14"

]

const gregorian = [

   "22 September 1792", "20 May 1795", "15 July 1799", 
   "23 September 1803", "31 December 1805"

]

function testrepublicancalendar()

   println("French Republican to Gregorian")
   for s in republican
       println(lpad(s, 24), " => ", togregorian(s))
   end
   println("Gregorian to French Republican")
   for s in gregorian
       println(lpad(s, 24), " => ", torepublican(s))
   end

end

testrepublicancalendar()

</lang>

Output:

French Republican to Gregorian
         1 Vendémiaire 1 => (22, "September", 1792)
            1 Prairial 3 => (20, "May", 1795)
           27 Messidor 7 => (15, "July", 1799)
Fête de la Révolution 11 => (23, "September", 1803)
            10 Nivôse 14 => (31, "December", 1805)
Gregorian to French Republican
       22 September 1792 => (1, "Vendémiaire", 1)
             20 May 1795 => (1, "Prairial", 3)
            15 July 1799 => (27, "Messidor", 7)
       23 September 1803 => ("Fête de la Révolution", 11)
        31 December 1805 => (10, "Nivôse", 14)

Kotlin

<lang scala>// version 1.1.4-3

import java.time.format.DateTimeFormatter import java.time.LocalDate import java.time.temporal.ChronoUnit.DAYS

/* year = 1.. month = 1..13 day = 1..30 */ class FrenchRCDate(val year: Int, val month: Int, val day: Int) {

   init {
       require (year > 0 && month in 1..13)
       if (month < 13) require (day in 1..30)
       else {
           val leap = isLeapYear(year)
           require (day in (if (leap) 1..6 else 1..5))
       }
   }

   override fun toString() =
       if (month < 13) "$day ${months[month - 1]} $year"
       else "${intercal[day - 1]} $year"

   fun toLocalDate(): LocalDate {
       var sumDays = 0L
       for (i in 1 until year) sumDays += if (isLeapYear(i)) 366 else 365
       val dayInYear = (month - 1) * 30 + day - 1
       return introductionDate.plusDays(sumDays + dayInYear)
   }

   companion object {
       /* uses the 'continuous method' for years after 1805 */
       fun isLeapYear(y: Int): Boolean {
           val yy = y + 1
           return (yy % 4 == 0) && (yy % 100 != 0 || yy % 400 == 0)
       }

       fun parse(frcDate: String): FrenchRCDate {
           val splits = frcDate.trim().split(' ')
           if (splits.size == 3) {
               val month = months.indexOf(splits[1]) + 1
               require(month in 1..13)
               val year = splits[2].toIntOrNull() ?: 0
               require(year > 0)
               val monthLength = if (month < 13) 30 else if (isLeapYear(year)) 6 else 5
               val day = splits[0].toIntOrNull() ?: 0
               require(day in 1..monthLength)
               return FrenchRCDate(year, month, day)
           }
           else if (splits.size in 4..5) {
               val yearStr = splits[splits.lastIndex]
               val year = yearStr.toIntOrNull() ?: 0
               require(year > 0)
               val scDay = frcDate.trim().dropLast(yearStr.length + 1)
               val day = intercal.indexOf(scDay) + 1
               val maxDay = if (isLeapYear(year)) 6 else 5
               require (day in 1..maxDay)
               return FrenchRCDate(year, 13, day)
           }
           else throw IllegalArgumentException("Invalid French Republican date")
       }

       /* for convenience we treat 'Sansculottide' as an extra month with 5 or 6 days */
       val months = arrayOf(
           "Vendémiaire", "Brumaire", "Frimaire", "Nivôse", "Pluviôse", "Ventôse", "Germinal",
           "Floréal", "Prairial", "Messidor", "Thermidor", "Fructidor", "Sansculottide"
       )

       val intercal = arrayOf(
           "Fête de la vertu", "Fête du génie", "Fête du travail",
           "Fête de l'opinion", "Fête des récompenses", "Fête de la Révolution"
       )

       val introductionDate = LocalDate.of(1792, 9, 22)
   }

}

fun LocalDate.toFrenchRCDate(): FrenchRCDate {

   val daysDiff  = DAYS.between(FrenchRCDate.introductionDate, this).toInt() + 1
   if (daysDiff <= 0) throw IllegalArgumentException("Date can't be before 22 September 1792")
   var year = 1
   var startDay = 1
   while (true) {
       val endDay = startDay + if (FrenchRCDate.isLeapYear(year)) 365 else 364
       if (daysDiff in startDay..endDay) break
       year++
       startDay = endDay + 1
   }
   val remDays = daysDiff - startDay
   val month  = remDays / 30
   val day = remDays - month * 30
   return FrenchRCDate(year, month + 1, day + 1)

}

fun main(args: Array<String>) {

   val formatter = DateTimeFormatter.ofPattern("d MMMM yyyy")
   val dates = arrayOf("22 September 1792", "20 May 1795", "15 July 1799", "23 September 1803",
                       "31 December 1805", "18 March 1871", "25 August 1944", "19 September 2016",
                       "22 September 2017", "28 September 2017")
   val frcDates = Array<String>(dates.size) { "" }
   for ((i, date) in dates.withIndex()) {
       val thisDate = LocalDate.parse(date, formatter)
       val frcd = thisDate.toFrenchRCDate()
       frcDates[i] = frcd.toString()
       println("${date.padEnd(25)} => $frcd")
   }

   // now process the other way around
   println()
   for (frcDate in frcDates) {
       val thisDate = FrenchRCDate.parse(frcDate)
       val lds = formatter.format(thisDate.toLocalDate())
       println("${frcDate.padEnd(25)} => $lds")
   }

}</lang>

Output:

22 September 1792         => 1 Vendémiaire 1
20 May 1795               => 1 Prairial 3
15 July 1799              => 27 Messidor 7
23 September 1803         => Fête de la Révolution 11
31 December 1805          => 10 Nivôse 14
18 March 1871             => 27 Ventôse 79
25 August 1944            => 7 Fructidor 152
19 September 2016         => Fête du travail 224
22 September 2017         => 1 Vendémiaire 226
28 September 2017         => 7 Vendémiaire 226

1 Vendémiaire 1           => 22 September 1792
1 Prairial 3              => 20 May 1795
27 Messidor 7             => 15 July 1799
Fête de la Révolution 11  => 23 September 1803
10 Nivôse 14              => 31 December 1805
27 Ventôse 79             => 18 March 1871
7 Fructidor 152           => 25 August 1944
Fête du travail 224       => 19 September 2016
1 Vendémiaire 226         => 22 September 2017
7 Vendémiaire 226         => 28 September 2017

Nim

As was done in Julia version, we have limited the date range to historical dates only. <lang Nim> import strformat, strscans, strutils, times

const

 RcMonths = ["Vendémiaire", "Brumaire", "Frimaire", "Nivôse", "Pluviôse", "Ventôse",
             "Germinal", "Floréal", "Prairial", "Messidor", "Thermidor", "Fructidor"]

 SansCulottides = ["Fête de la vertu", "Fête du génie", "Fête du travail",
                   "Fête de l’opinion", "Fête des récompenses", "Fête de la Révolution"]

let

 # First and last dates of republican calendar expressed in gregorian calendar.
 FirstRcDate = initDateTime(22, mSep, 1792, 0, 0, 0)
 LastRcDate = initDateTime(31, mDec, 1805, 0, 0, 0)

type

 # French republican date representation.
 RcDayRange = 1..30
 RcMonthRange = 1..13
 RcYearRange = 1..14
 RepublicanDate = tuple[year: RcYearRange, month: RcMonthRange, day: RcDayRange]

1. Last dates of republican calendar expressed in republican calendar.

const RcLastDate: RepublicanDate = (RcYearRange(14), RcMonthRange(4), RcDayRange(10))

proc notnum(input: string; str: var string; start: int): int =

 # Parsing procedure to extract non numerical part of a date.
 var i = start
 while i <= input.high:
   if input[i] in '0'..'9': break
   str.add input[i]
   inc i
 if str.len == 0 or str[^1] != ' ': return -1   # Not terminated by a space.
 str.setLen(str.len - 1)   # Back before the space.
 result = str.len

proc parseRepublicanDate(rdate: string): RepublicanDate =

 ## Parse a French republican date and return its representation.

 let date = rdate.strip()
 var day, month, year: int
 var monthString, dayString: string

 if date.scanf("$i $+ $i", day, monthString, year):
   # Normal day.
   if day notin 1..30:
     raise newException(ValueError, "wrong day number: $1.".format(day))
   month = RcMonths.find(monthString) + 1
   if month == 0:
     raise newException(ValueError, "unknown French republican month: $1." % monthString)

 elif date.scanf("${notnum} $i", dayString, year):
   # Sans-culottide day (also known as “jour complémentaire”).
   month = 13  # Value used for sans-culottide days.
   day = SansCulottides.find(dayString) + 1
   if day == 0:
     raise newException(ValueError, "wrong “sans-culottide” day: « $1 »." % dayString)
   if day == 6 and year mod 4 != 3:
     raise newException(ValueError, "republican year $1 is not a leap year".format(year))

 else:
   raise newException(ValueError, "invalid French republican date: « $1 »." % date)

 result = (RcYearRange(year), RcMonthRange(month), RcDayRange(day))
 if result > RcLastDate:
   raise newException(ValueError, "republican date out of range: « $1 »." % date)

proc `$`(date: RepublicanDate): string =

 ## Return the string representation of a French republican date.

 if date.month != 13:
   # Normal day.
   result = "$1 $2 $3".format(date.day, RcMonths[date.month - 1], date.year)
 else:
   # Supplementary day.
   result = "$1 $2".format(SansCulottides[date.day - 1], date.year)

proc toGregorian(rdate: RepublicanDate): DateTime =

 ## Convert a republican date tuple to a gregorian date (DateTime object).
 let day = (rdate.day - 1) + (rdate.month - 1) * 30 + (rdate.year - 1) * 365 + rdate.year div 4
 result = FirstRcDate + initTimeInterval(days = day)

proc toGregorian(rdate: string): string =

 ## Convert a republican date string to a gregorian date string.
 let date = rdate.parseRepublicanDate()
 result = date.toGregorian().format("dd MMMM yyyy")

proc toRepublican(gdate: DateTime): RepublicanDate =

 ## Convert a gregorian date (DateTime object) to a republican date tuple.

 if gdate notin FirstRcDate..LastRcDate:
   raise newException(ValueError, "impossible conversion to republican date.")
 let d = gdate - FirstRcDate

 # Add a dummy year before year 1 in order to use a four years period.
 let dayNumber = d.inDays + 365
 let periodNum = dayNumber div 1461
 let dayInPeriod = dayNumber mod 1461

 # Compute year and day in year.
 let yearInPeriod = min(dayInPeriod div 365, 3)
 result.year = periodNum * 4 + yearInPeriod
 let dayInYear = dayInPeriod - yearInPeriod * 365

 # Compute month and day.
 result.month = dayInYear div 30 + 1
 result.day = dayInYear mod 30 + 1

proc toRepublican(gdate: string): string =

 ## Convert a gregorian date string to a republican date string.
 let date = gdate.parse("d MMMM yyyy")
 result = $(date.toRepublican())

when isMainModule:

 const
   RepublicanDates = ["1 Vendémiaire 1", "1 Prairial 3", "27 Messidor 7",
                     "Fête de la Révolution 11", "10 Nivôse 14"]
   GregorianDates = ["22 September 1792", "20 May 1795", "15 July 1799",
                     "23 September 1803", "31 December 1805"]

 echo "From French republican dates to gregorian dates:"
 for rdate in RepublicanDates:
   echo &"{rdate:>24} → {rdate.toGregorian()}"
 echo()

 echo "From gregorian dates to French republican dates:"
 for gdate in GregorianDates:
   echo &"{gdate:>24} → {gdate.toRepublican()}"</lang>

Output:

From French republican dates to gregorian dates:
         1 Vendémiaire 1 → 22 September 1792
            1 Prairial 3 → 20 May 1795
           27 Messidor 7 → 15 July 1799
Fête de la Révolution 11 → 23 September 1803
            10 Nivôse 14 → 31 December 1805

From gregorian dates to French republican dates:
       22 September 1792 → 1 Vendémiaire 1
             20 May 1795 → 1 Prairial 3
            15 July 1799 → 27 Messidor 7
       23 September 1803 → Fête de la Révolution 11
        31 December 1805 → 10 Nivôse 14

Perl

<lang perl>use feature 'state'; use DateTime; my @month_names = qw{

   Vendémiaire Brumaire Frimaire  Nivôse   Pluviôse  Ventôse
   Germinal    Floréal  Prairial  Messidor Thermidor Fructidor

}; my @intercalary = (

   'Fête de la vertu',  'Fête du génie',        'Fête du travail',
   "Fête de l'opinion", 'Fête des récompenses', 'Fête de la Révolution',

); my %month_nums = map { $month_names[$_] => $_+1 } 0 .. $#month_names; my %i_cal_nums = map { $intercalary[$_] => $_+1 } 0 .. $#intercalary; my $i_cal_month = 13; my $epoch = DateTime->new( year => 1792, month => 9, day => 22 );

sub is_republican_leap_year {

   my $y = $_[0] + 1;
   return !!( ($y % 4)==0 and (($y % 100)!=0 or ($y % 400)==0) );

}

sub Republican_to_Gregorian {

   my ($rep_date) = @_;
   state $months   = join '|', map { quotemeta } @month_names;
   state $intercal = join '|', map { quotemeta } @intercalary;
   state $re = qr{
     \A
       \s* (?:
               (?<ic> $intercal)
             | (?<day> \d+) \s+ (?<month> $months)
           )
       \s+ (?<year> \d+)
       \s*
     \z
   }msx;

   $rep_date =~ /$re/
       or die "Republican date not recognized: '$rep_date'";

   my $day1   = $+{ic}    ? $i_cal_nums{$+{ic}}    : $+{day};
   my $month1 = $+{month} ? $month_nums{$+{month}} : $i_cal_month;
   my $year1  = $+{year};

   my $days_since_epoch = ($year1-1) * 365 + ($month1-1) * 30 + ($day1-1);

   my $leap_days = grep { is_republican_leap_year($_) } 1 .. $year1-1;
   return $epoch->clone->add( days => ($days_since_epoch + $leap_days) );

}

sub Gregorian_to_Republican {

   my ($greg_date) = @_;

   my $days_since_epoch = $epoch->delta_days($greg_date)->in_units('days');
   die if $days_since_epoch < 0;
   my ( $year, $days ) = ( 1, $days_since_epoch );
   while (1) {
       my $year_length = 365 + ( is_republican_leap_year($year) ? 1 : 0 );
       last if $days < $year_length;
       $days -= $year_length;
       $year += 1;
   }
   my $day0   = $days % 30;
   my $month0 = ($days - $day0) / 30;

   my ( $day1, $month1 ) = ( $day0 + 1, $month0 + 1 );

   return $month1 == $i_cal_month
       ?       "$intercalary[$day0  ] $year"
       : "$day1 $month_names[$month0] $year";

}

while () {

   s{\s*\#.+\n?\z}{};
   /^(\d{4})-(\d{2})-(\d{2})\s+(\S.+?\S)\s*$/ or die;

   my $g = DateTime->new( year => $1, month => $2, day => $3 );
   my $r = $4;

   die if Republican_to_Gregorian($r) != $g
       or Gregorian_to_Republican($g) ne $r;

   die if Gregorian_to_Republican(Republican_to_Gregorian($r)) ne $r
       or Republican_to_Gregorian(Gregorian_to_Republican($g)) != $g;

} say 'All tests successful.';

__DATA__ 1792-09-22 1 Vendémiaire 1 1795-05-20 1 Prairial 3 1799-07-15 27 Messidor 7 1803-09-23 Fête de la Révolution 11 1805-12-31 10 Nivôse 14 1871-03-18 27 Ventôse 79 1944-08-25 7 Fructidor 152 2016-09-19 Fête du travail 224 1871-05-06 16 Floréal 79 # Paris Commune begins 1871-05-23 3 Prairial 79 # Paris Commune ends 1799-11-09 18 Brumaire 8 # Revolution ends by Napoléon coup 1804-12-02 11 Frimaire 13 # Republic ends by Napoléon coronation 1794-10-30 9 Brumaire 3 # École Normale Supérieure established 1794-07-27 9 Thermidor 2 # Robespierre falls 1799-05-27 8 Prairial 7 # Fromental Halévy born 1792-09-22 1 Vendémiaire 1 1793-09-22 1 Vendémiaire 2 1794-09-22 1 Vendémiaire 3 1795-09-23 1 Vendémiaire 4 1796-09-22 1 Vendémiaire 5 1797-09-22 1 Vendémiaire 6 1798-09-22 1 Vendémiaire 7 1799-09-23 1 Vendémiaire 8 1800-09-23 1 Vendémiaire 9 1801-09-23 1 Vendémiaire 10 1802-09-23 1 Vendémiaire 11 1803-09-24 1 Vendémiaire 12 1804-09-23 1 Vendémiaire 13 1805-09-23 1 Vendémiaire 14 1806-09-23 1 Vendémiaire 15 1807-09-24 1 Vendémiaire 16 1808-09-23 1 Vendémiaire 17 1809-09-23 1 Vendémiaire 18 1810-09-23 1 Vendémiaire 19 1811-09-24 1 Vendémiaire 20 2015-09-23 1 Vendémiaire 224 2016-09-22 1 Vendémiaire 225 2017-09-22 1 Vendémiaire 226 </lang>

Output:

All tests successful.

Phix

with javascript_semantics
constant gregorians = {"January", "February", "March", "April", "May", "June", "July", 
                       "August", "September", "October", "November", "December"},
         gregorian = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31},
         republicans = {"Vendémiaire", "Brumaire", "Frimaire", "Nivôse", "Pluviôse", 
                        "Ventôse", "Germinal", "Floréal", "Prairial", "Messidor", 
                        "Thermidor", "Fructidor"},
         sansculottides = {"Fête de la vertu", "Fête du génie", "Fête du travail",
                           "Fête de l'opinion", "Fête des récompenses",
                           "Fête de la Révolution"}
 
function rep_leap(integer year)
    return mod(year+1,4)==0 and (mod(year+1,100)!=0 or mod(year+1,400)==0)
end function
 
function rep_to_day(sequence dmy)
    integer {d, m, y} = dmy
    if m == 13 then
        m -= 1
        d += 30
    end if
    if rep_leap(y) then
        d -= 1
    end if
    integer res = 365*(y-1) + floor((y+1)/4) - floor((y+1)/100) + floor((y+1)/400) + 30*(m-1) + d
    return res
end function
 
function gre_leap(integer year)
    return mod(year,4)==0 and (mod(year,100)!=0 or mod(year,400)==0)
end function
 
function gre_to_day(sequence dmy)
    integer {d, m, y} = dmy
    if m < 3 then
        y -= 1
        m += 12
    end if
    integer res =  floor(y*365.25) - floor(y/100) + floor(y/400)
                 + floor(30.6*(m+1)) + d - 654842
    return res
end function
 
function day_to_rep(integer day)
    integer y = floor((day-1)/365.25)
    if not rep_leap(y) then
        y += 1
    end if
    integer d = day - floor(y*365.25) + 365 + floor(y/100) - floor(y/400),
            m = 1,
            sansculottide = 5+rep_leap(y)
    while d>30 do
        d -= 30
        m += 1
        if m == 13 then
            if d > sansculottide then
                d -= sansculottide
                m = 1
                y += 1
                sansculottide = 5 + rep_leap(y)
            end if
        end if
    end while
    return {d,m,y}
end function
 
function day_to_gre(integer day)
    integer y = floor(day/365.25),
            d = day - floor(y*365.25) + 21,
            m = 9
    y += 1792
    d += floor(y/100) - floor(y/400) - 13
    sequence gregoriam = deep_copy(gregorian)  -- (modifiable copy)
    while d>gregoriam[m] do
        d -= gregoriam[m]
        m += 1
        if m == 13 then
            m = 1
            y += 1
            gregoriam[2] = 28 + gre_leap(y)
        end if
    end while
    return {d,m,y}
end function
 
function greg_to_frep(string greg)
    {integer day, string months, integer year} = scanf(greg,"%d %s %d")[1]
    integer month = find(months,gregorians)
    {day,month,year} = day_to_rep(gre_to_day({day,month,year}))
    string frep = iff(month=13?sprintf("%s %d",{sansculottides[day], year})
                              :sprintf("%d %s %d",{day, republicans[month], year}))
    return frep
end function    
 
function frep_to_greg(string frep)
    integer day, month, year
    string months, days
    if frep[1]<='9' then
        {day, months, year} = scanf(frep,"%d %s %d")[1]
        month = find(months,republicans)
    else
        {days, year} = scanf(frep,"%s %d")[1]
        day = find(days,sansculottides)
        month = 13
    end if
    {day,month,year} = day_to_gre(rep_to_day({day, month, year}))
    string greg = sprintf("%02d %s %d",{day, gregorians[month], year})
    return greg
end function
 
constant test_data = {
                      { "22 September 1792", "1 Vendémiaire 1" },
                      { "20 May 1795", "1 Prairial 3" },
                      { "15 July 1799", "27 Messidor 7" },
                      { "23 September 1803", "Fête de la Révolution 11" },
                      { "31 December 1805", "10 Nivôse 14" },
 
                      { "18 March 1871", "27 Ventôse 79" },
                      { "25 August 1944", "7 Fructidor 152" },
                      { "19 September 2016", "Fête du travail 224" },
 
                      { "06 May 1871", "16 Floréal 79" },       -- Paris Commune begins
                      { "23 May 1871", "3 Prairial 79" },       -- Paris Commune ends
                      { "09 November 1799", "18 Brumaire 8" },  -- Revolution ends by Napoléon coup
                      { "02 December 1804", "11 Frimaire 13" }, -- Republic ends by Napoléon coronation
                      { "30 October 1794", "9 Brumaire 3" },    -- École Normale Supérieure established
                      { "27 July 1794", "9 Thermidor 2" },      -- Robespierre falls
                      { "27 May 1799", "8 Prairial 7" },        -- Fromental Halévy born
 
                      { "22 September 1792", "1 Vendémiaire 1" },
                      { "22 September 1793", "1 Vendémiaire 2" },
                      { "22 September 1794", "1 Vendémiaire 3" },
                      { "23 September 1795", "1 Vendémiaire 4" },
                      { "22 September 1796", "1 Vendémiaire 5" },
                      { "22 September 1797", "1 Vendémiaire 6" },
                      { "22 September 1798", "1 Vendémiaire 7" },
                      { "23 September 1799", "1 Vendémiaire 8" },
                      { "23 September 1800", "1 Vendémiaire 9" },
                      { "23 September 1801", "1 Vendémiaire 10" },
                      { "23 September 1802", "1 Vendémiaire 11" },
                      { "24 September 1803", "1 Vendémiaire 12" },
                      { "23 September 1804", "1 Vendémiaire 13" },
                      { "23 September 1805", "1 Vendémiaire 14" },
                      { "23 September 1806", "1 Vendémiaire 15" },
                      { "24 September 1807", "1 Vendémiaire 16" },
                      { "23 September 1808", "1 Vendémiaire 17" },
                      { "23 September 1809", "1 Vendémiaire 18" },
                      { "23 September 1810", "1 Vendémiaire 19" },
                      { "24 September 1811", "1 Vendémiaire 20" },
                      { "23 September 2015", "1 Vendémiaire 224" },
                      { "21 September 2016", "Fête des récompenses 224" },
                      { "22 September 2016", "1 Vendémiaire 225" },
                      { "23 September 2016", "2 Vendémiaire 225" },
                      { "22 September 2017", "1 Vendémiaire 226" },
                      { "28 September 2017", "7 Vendémiaire 226" } }
 
for i=1 to length(test_data) do
    string {greg, frep} = test_data[i],
           frep2 = greg_to_frep(greg),
           greg2 = frep_to_greg(frep),
           ok = iff(frep=frep2 and greg=greg2?"ok":"**** ERROR ****")
    if platform()=WINDOWS then
        -- the windows console does not handle 
        -- non-basic-latin-ascii characters well...
        frep = substitute_all(frep,"éêéô","eeeo")
    end if
    printf(1,"%18s <==> %-25s %s\n",{greg,frep,ok})
end for
 
--sanity test:
for i=1 to 150000 do -- (years 1792..~2203)
    if rep_to_day(day_to_rep(i))!=i then ?9/0 end if
    if gre_to_day(day_to_gre(i))!=i then ?9/0 end if
end for

Output:

 22 September 1792 <==> 1 Vendemiaire 1           ok
       20 May 1795 <==> 1 Prairial 3              ok
      15 July 1799 <==> 27 Messidor 7             ok
 23 September 1803 <==> Fete de la Revolution 11  ok
  31 December 1805 <==> 10 Nivose 14              ok
     18 March 1871 <==> 27 Ventose 79             ok
    25 August 1944 <==> 7 Fructidor 152           ok
 19 September 2016 <==> Fete du travail 224       ok
       06 May 1871 <==> 16 Floreal 79             ok
       23 May 1871 <==> 3 Prairial 79             ok
  09 November 1799 <==> 18 Brumaire 8             ok
  02 December 1804 <==> 11 Frimaire 13            ok
   30 October 1794 <==> 9 Brumaire 3              ok
      27 July 1794 <==> 9 Thermidor 2             ok
       27 May 1799 <==> 8 Prairial 7              ok
 22 September 1792 <==> 1 Vendemiaire 1           ok
 22 September 1793 <==> 1 Vendemiaire 2           ok
 22 September 1794 <==> 1 Vendemiaire 3           ok
 23 September 1795 <==> 1 Vendemiaire 4           ok
 22 September 1796 <==> 1 Vendemiaire 5           ok
 22 September 1797 <==> 1 Vendemiaire 6           ok
 22 September 1798 <==> 1 Vendemiaire 7           ok
 23 September 1799 <==> 1 Vendemiaire 8           ok
 23 September 1800 <==> 1 Vendemiaire 9           ok
 23 September 1801 <==> 1 Vendemiaire 10          ok
 23 September 1802 <==> 1 Vendemiaire 11          ok
 24 September 1803 <==> 1 Vendemiaire 12          ok
 23 September 1804 <==> 1 Vendemiaire 13          ok
 23 September 1805 <==> 1 Vendemiaire 14          ok
 23 September 1806 <==> 1 Vendemiaire 15          ok
 24 September 1807 <==> 1 Vendemiaire 16          ok
 23 September 1808 <==> 1 Vendemiaire 17          ok
 23 September 1809 <==> 1 Vendemiaire 18          ok
 23 September 1810 <==> 1 Vendemiaire 19          ok
 24 September 1811 <==> 1 Vendemiaire 20          ok
 23 September 2015 <==> 1 Vendemiaire 224         ok
 21 September 2016 <==> Fete des recompenses 224  ok
 22 September 2016 <==> 1 Vendemiaire 225         ok
 23 September 2016 <==> 2 Vendemiaire 225         ok
 22 September 2017 <==> 1 Vendemiaire 226         ok
 28 September 2017 <==> 7 Vendemiaire 226         ok

Raku

(formerly Perl 6) <lang perl6>use v6; constant @month_names = <

   Vendémiaire Brumaire Frimaire  Nivôse   Pluviôse  Ventôse
   Germinal    Floréal  Prairial  Messidor Thermidor Fructidor

>; constant @intercalary =

   'Fête de la vertu',  'Fête du génie',        'Fête du travail',
   "Fête de l'opinion", 'Fête des récompenses', 'Fête de la Révolution',

constant %month_nums = %( @month_names Z=> 1..12 ); constant %i_cal_nums = %( @intercalary Z=> 1.. 6 ); constant $i_cal_month = 13; constant $epoch = Date.new: '1792-09-22';

sub is_republican_leap_year ( Int:D $year --> Bool ) {

   my $y := $year + 1;
   return ?( $y %% 4 and ($y !%% 100 or $y %% 400) );

}

sub Republican_to_Gregorian ( Str:D $rep_date --> Date ) {

   grammar Republican_date_text {
       token day   { \d+ }
       token year  { \d+ }
       token ic    { @intercalary }
       token month { @month_names }
       rule TOP    { ^ [ <ic> | <day> <month> ] <year> $ }
   }
   
   Republican_date_text.parse($rep_date)
       orelse die "Republican date not recognized: '$rep_date'";

   my $day1   := $/<ic>    ?? %i_cal_nums{~$/<ic>}    !! +$/<day>;
   my $month1 := $/<month> ?? %month_nums{~$/<month>} !! $i_cal_month;

   my @ymd0 := ($/<year>, $month1, $day1) »-» 1;
   my $days_since_epoch := [+] @ymd0 Z* (365, 30, 1);

   my $leap_days := +grep &is_republican_leap_year, 1 ..^ $/<year>;
   return $epoch + $days_since_epoch + $leap_days;

}

sub Gregorian_to_Republican ( Date:D $greg_date --> Str ) {

   my $days_since_epoch := $greg_date - $epoch;
   die if $days_since_epoch < 0;

   my ( $year, $days ) = 1, $days_since_epoch;
   loop {
       my $year_length = 365 + (1 if $year.&is_republican_leap_year);
       last if $days < $year_length;
       $days -= $year_length;
       $year += 1;
   }

   my ( $day0, $month0 ) = $days.polymod( 30 );
   my ( $day1, $month1 ) = ($day0, $month0) X+ 1;

   return $month1 == $i_cal_month
       ??       "@intercalary[$day0  ] $year"
       !! "$day1 @month_names[$month0] $year";

}

my @test_data =

   ( '1792-09-22', '1 Vendémiaire 1' ),
   ( '1795-05-20', '1 Prairial 3' ),
   ( '1799-07-15', '27 Messidor 7' ),
   ( '1803-09-23', 'Fête de la Révolution 11' ),
   ( '1805-12-31', '10 Nivôse 14' ),

   ( '1871-03-18', '27 Ventôse 79' ),
   ( '1944-08-25', '7 Fructidor 152' ),
   ( '2016-09-19', 'Fête du travail 224' ),

   ( '1871-05-06', '16 Floréal 79'  ), # Paris Commune begins
   ( '1871-05-23', '3 Prairial 79'  ), # Paris Commune ends
   ( '1799-11-09', '18 Brumaire 8'  ), # Revolution ends by Napoléon coup
   ( '1804-12-02', '11 Frimaire 13' ), # Republic   ends by Napoléon coronation
   ( '1794-10-30', '9 Brumaire 3'   ), # École Normale Supérieure established
   ( '1794-07-27', '9 Thermidor 2'  ), # Robespierre falls
   ( '1799-05-27', '8 Prairial 7'   ), # Fromental Halévy born

   ( '1792-09-22', '1 Vendémiaire 1'   ),
   ( '1793-09-22', '1 Vendémiaire 2'   ),
   ( '1794-09-22', '1 Vendémiaire 3'   ),
   ( '1795-09-23', '1 Vendémiaire 4'   ),
   ( '1796-09-22', '1 Vendémiaire 5'   ),
   ( '1797-09-22', '1 Vendémiaire 6'   ),
   ( '1798-09-22', '1 Vendémiaire 7'   ),
   ( '1799-09-23', '1 Vendémiaire 8'   ),
   ( '1800-09-23', '1 Vendémiaire 9'   ),
   ( '1801-09-23', '1 Vendémiaire 10'  ),
   ( '1802-09-23', '1 Vendémiaire 11'  ),
   ( '1803-09-24', '1 Vendémiaire 12'  ),
   ( '1804-09-23', '1 Vendémiaire 13'  ),
   ( '1805-09-23', '1 Vendémiaire 14'  ),
   ( '1806-09-23', '1 Vendémiaire 15'  ),
   ( '1807-09-24', '1 Vendémiaire 16'  ),
   ( '1808-09-23', '1 Vendémiaire 17'  ),
   ( '1809-09-23', '1 Vendémiaire 18'  ),
   ( '1810-09-23', '1 Vendémiaire 19'  ),
   ( '1811-09-24', '1 Vendémiaire 20'  ),
   ( '2015-09-23', '1 Vendémiaire 224' ),
   ( '2016-09-22', '1 Vendémiaire 225' ),
   ( '2017-09-22', '1 Vendémiaire 226' ),

for @test_data -> ( $g_text, $r ) {

   my $g = Date.new: $g_text;

   die if Republican_to_Gregorian($r) != $g
       or Gregorian_to_Republican($g) ne $r;

   die if Gregorian_to_Republican(Republican_to_Gregorian($r)) ne $r
       or Republican_to_Gregorian(Gregorian_to_Republican($g)) != $g;

} say 'All tests successful.';</lang>

Output:

All tests successful.

Sidef

Translation of: Perl

<lang ruby>require('DateTime')

var month_names = %w(

 Vendémiaire Brumaire Frimaire  Nivôse   Pluviôse  Ventôse
 Germinal    Floréal  Prairial  Messidor Thermidor Fructidor

)

var intercalary = [

                  'Fête de la vertu',
                  'Fête du génie',
                  'Fête du travail',
                  "Fête de l'opinion",
                  'Fête des récompenses',
                  'Fête de la Révolution',
                 ]

var i_cal_month = 13 var epoch = %O<DateTime>.new(year => 1792, month => 9, day => 22)

var month_nums = Hash(month_names.kv.map {|p| [p[1], p[0]+1] }.flat...) var i_cal_nums = Hash(intercalary.kv.map {|p| [p[1], p[0]+1] }.flat...)

func is_republican_leap_year(Number year) -> Bool {

   var y = (year + 1)
   !!(4.divides(y) && (!100.divides(y) || 400.divides(y)))

}

func Republican_to_Gregorian(String rep_date) -> String {

   static months   = month_names.map { .escape }.join('|')
   static intercal = intercalary.map { .escape }.join('|')
   static re       = Regex("^
       \\s* (?:
               (?<ic> #{intercal})
             | (?<day> \\d+) \\s+ (?<month> #{months})
           )
       \\s+ (?<year> \\d+)
       \\s*
     \\z", 'x')

   var m = (rep_date =~ re)
   m || die "Republican date not recognized: '#{rep_date}'"

   var ncap = m.named_captures

   var day1   = Number(ncap{:ic}    ? i_cal_nums{ncap{:ic}}    : ncap{:day})
   var month1 = Number(ncap{:month} ? month_nums{ncap{:month}} : i_cal_month)
   var year1  = Number(ncap{:year})

   var days_since_epoch = (365*(year1 - 1) + 30*(month1 - 1) + (day1 - 1))

   var leap_days = (1 ..^ year1 -> grep { is_republican_leap_year(_) })
   epoch.clone.add(days => (days_since_epoch + leap_days)).strftime("%Y-%m-%d")

}

func Gregorian_to_Republican(String greg_date) -> String {

   var m = (greg_date =~ /^(\d{4})-(\d{2})-(\d{2})\z/)
   m || die "Gregorian date not recognized: '#{greg_date}'"

   var g = %O<DateTime>.new(year => m[0], month => m[1], day => m[2])
   var days_since_epoch = epoch.delta_days(g).in_units('days')
   days_since_epoch < 0 && die "unexpected error"
   var (year, days) = (1, days_since_epoch)

   loop {
       var year_length = (365 + (is_republican_leap_year(year) ? 1 : 0))
       days < year_length && break
       days -= year_length
       year += 1;
   }

   var day0   = (days % 30)
   var month0 = (days - day0)/30

   var (day1, month1) = (day0 + 1, month0 + 1)

   (month1 == i_cal_month
       ? "#{intercalary[day0]} #{year}"
       : "#{day1} #{month_names[month0]} #{year}")

}

for line in DATA {

   line.sub!(/\s*\#.+\R?\z/, )

   var m = (line =~ /^(\d{4})-(\d{2})-(\d{2})\s+(\S.+?\S)\s*$/)
   m || die "error for: #{line.dump}"

   var g = "#{m[0]}-#{m[1]}-#{m[2]}"
   var r = m[3]

   var r2g = Republican_to_Gregorian(r)
   var g2r = Gregorian_to_Republican(g)

   #say "#{r} -> #{r2g}"
   #say "#{g} -> #{g2r}"

   if ((g2r != r) || (r2g != g)) {
       die "1-way error"
   }

   if ((Gregorian_to_Republican(r2g) != r) ||
       (Republican_to_Gregorian(g2r) != g)
   ) {
       die "2-way error"
   }

}

say 'All tests successful.'

__DATA__ 1792-09-22 1 Vendémiaire 1 1795-05-20 1 Prairial 3 1799-07-15 27 Messidor 7 1803-09-23 Fête de la Révolution 11 1805-12-31 10 Nivôse 14 1871-03-18 27 Ventôse 79 1944-08-25 7 Fructidor 152 2016-09-19 Fête du travail 224 1871-05-06 16 Floréal 79 # Paris Commune begins 1871-05-23 3 Prairial 79 # Paris Commune ends 1799-11-09 18 Brumaire 8 # Revolution ends by Napoléon coup 1804-12-02 11 Frimaire 13 # Republic ends by Napoléon coronation 1794-10-30 9 Brumaire 3 # École Normale Supérieure established 1794-07-27 9 Thermidor 2 # Robespierre falls 1799-05-27 8 Prairial 7 # Fromental Halévy born 1792-09-22 1 Vendémiaire 1 1793-09-22 1 Vendémiaire 2 1794-09-22 1 Vendémiaire 3 1795-09-23 1 Vendémiaire 4 1796-09-22 1 Vendémiaire 5 1797-09-22 1 Vendémiaire 6 1798-09-22 1 Vendémiaire 7 1799-09-23 1 Vendémiaire 8 1800-09-23 1 Vendémiaire 9 1801-09-23 1 Vendémiaire 10 1802-09-23 1 Vendémiaire 11 1803-09-24 1 Vendémiaire 12 1804-09-23 1 Vendémiaire 13 1805-09-23 1 Vendémiaire 14 1806-09-23 1 Vendémiaire 15 1807-09-24 1 Vendémiaire 16 1808-09-23 1 Vendémiaire 17 1809-09-23 1 Vendémiaire 18 1810-09-23 1 Vendémiaire 19 1811-09-24 1 Vendémiaire 20 2015-09-23 1 Vendémiaire 224 2016-09-22 1 Vendémiaire 225 2017-09-22 1 Vendémiaire 226</lang>

Output:

All tests successful.

Wren

Translation of: Kotlin

Library: Wren-date

Library: Wren-seq

Library: Wren-fmt

<lang ecmascript>import "/date" for Date import "/seq" for Lst import "/fmt" for Fmt

class FrenchRCDate {

   /* uses the 'continuous method' for years after 1805 */
   static isLeapYear(y) {
       var yy = y + 1
        return (yy % 4 == 0) && (yy % 100 != 0 || yy % 400 == 0)
   }

   static parse(frcDate) {
       var splits = frcDate.trim().split(" ")
       if (splits.count == 3) {
           var month = Lst.indexOf(months, splits[1]) + 1
           if (month < 1 || month > 13) Fiber.abort("Invalid month.")
           var year = Num.fromString(splits[2])
           if (year < 1) Fiber.abort("Invalid year.")
           var monthLength = (month < 13) ? 30 : (isLeapYear(year) ? 6 : 5)
           var day = Num.fromString(splits[0])
           if (day < 1 || day > monthLength) Fiber.abort("Invalid day.")
           return FrenchRCDate.new(year, month, day)
       } else if (splits.count == 4 || splits.count == 5) {
           var yearStr = splits[-1]
           var year = Num.fromString(yearStr)
           if (year < 1) Fiber.abort("Invalid year.")
           var scDay = frcDate.trim()[0...-(yearStr.count + 1)]
           var day = Lst.indexOf(intercal, scDay) + 1
           var maxDay = isLeapYear(year) ? 6 : 5
           if (day < 1 || day > maxDay) Fiber.abort("Invalid day.")
           return FrenchRCDate.new(year, 13, day)
        } else Fiber.abort("Invalid French Republican date.")
   }

   /* for convenience we treat 'Sansculottide' as an extra month with 5 or 6 days */
   static months {
       return ["Vendémiaire", "Brumaire", "Frimaire", "Nivôse", "Pluviôse", "Ventôse", "Germinal",
               "Floréal", "Prairial", "Messidor", "Thermidor", "Fructidor", "Sansculottide"]
   }

   static intercal {
       return ["Fête de la vertu", "Fête du génie", "Fête du travail",
               "Fête de l'opinion", "Fête des récompenses", "Fête de la Révolution"]
   }

   static introductionDate { Date.new(1792, 9, 22) }

   /* year = 1..  month = 1..13  day = 1..30 */
   construct new(year, month, day) {
       if (year <= 0 || month < 1 || month > 13) Fiber.abort("Invalid date.")
       if (month < 13) {
           if (day < 1 || day > 30)  Fiber.abort("Invalid date.")
       } else {
           var leap = FrenchRCDate.isLeapYear(year)
           if (leap  && (day < 1 || day > 6)) Fiber.abort("Invalid date.")
           if (!leap && (day < 1 || day > 5)) Fiber.abort("Invalid date.")
       }
       _year = year
       _month = month
       _day = day
   }

   static fromLocalDate(ldate) {
       var daysDiff = (ldate - introductionDate).days + 1
       if (daysDiff <= 0) Fiber.abort("Date can't be before 22 September 1792.")
       var year = 1
       var startDay = 1
       while (true) {
           var endDay = startDay + (isLeapYear(year) ? 365 : 364)
           if (daysDiff >= startDay && daysDiff <= endDay) break
           year = year + 1
           startDay = endDay + 1
       }
       var remDays = daysDiff - startDay
       var month  = (remDays / 30).floor
       var day = remDays - month * 30
       return FrenchRCDate.new(year, month + 1, day + 1)
   }

   toString {
       if (_month < 13) return "%(_day) %(FrenchRCDate.months[_month - 1]) %(_year)"
       return "%(FrenchRCDate.intercal[_day - 1]) %(_year)"
   }

   toLocalDate {
       var sumDays = 0
       for (i in 1..._year) sumDays = sumDays + (FrenchRCDate.isLeapYear(i) ? 366 : 365)
       var dayInYear = (_month - 1) * 30 + _day - 1
       return FrenchRCDate.introductionDate.addDays(sumDays + dayInYear)
   }

}

var fmt = "d| |mmmm| |yyyy" var dates = [

    "22 September 1792", "20 May 1795", "15 July 1799", "23 September 1803",
    "31 December 1805", "18 March 1871", "25 August 1944", "19 September 2016",
    "22 September 2017", "28 September 2017"

] var frcDates = List.filled(dates.count, null) var i = 0 for (date in dates) {

   var thisDate = Date.parse(date, fmt)
   var frcd = FrenchRCDate.fromLocalDate(thisDate)
   frcDates[i] = frcd.toString
   Fmt.print("$-25s => $s", date, frcd)
   i = i + 1

}

// now process the other way around System.print() for (frcDate in frcDates) {

   var thisDate = FrenchRCDate.parse(frcDate)
   var lds = thisDate.toLocalDate.format(fmt)
   Fmt.print("$-25s => $s", frcDate, lds)

}</lang>

Output:

22 September 1792         => 1 Vendémiaire 1
20 May 1795               => 1 Prairial 3
15 July 1799              => 27 Messidor 7
23 September 1803         => Fête de la Révolution 11
31 December 1805          => 10 Nivôse 14
18 March 1871             => 27 Ventôse 79
25 August 1944            => 7 Fructidor 152
19 September 2016         => Fête du travail 224
22 September 2017         => 1 Vendémiaire 226
28 September 2017         => 7 Vendémiaire 226

1 Vendémiaire 1           => 22 September 1792
1 Prairial 3              => 20 May 1795
27 Messidor 7             => 15 July 1799
Fête de la Révolution 11  => 23 September 1803
10 Nivôse 14              => 31 December 1805
27 Ventôse 79             => 18 March 1871
7 Fructidor 152           => 25 August 1944
Fête du travail 224       => 19 September 2016
1 Vendémiaire 226         => 22 September 2017
7 Vendémiaire 226         => 28 September 2017