Days between dates

Days between dates is a draft programming task. It is not yet considered ready to be promoted as a complete task, for reasons that should be found in its talk page.

Task

Calculate the number of days between two dates. Date input should be of the form YYYY-MM-DD.

Motivation

To demonstrate one of the numerous ways this can be done.

Erlang

<lang erlang>

-module(daysbetween). -export([between/2,dateToInts/2]).

dateToInts(String,POS) ->

 list_to_integer(lists:nth(POS,re:split(String ,"-",[{return,list},trim]))).

between(DateOne,DateTwo) ->

 Y1 = dateToInts(DateOne ,1),
 M1 = dateToInts(DateOne ,2),
 D1 = dateToInts(DateOne ,3),
 Y2 = dateToInts(DateTwo ,1),
 M2 = dateToInts(DateTwo ,2),
 D2 = dateToInts(DateTwo ,3),
 GregOne = calendar:date_to_gregorian_days(Y1,M1,D1),
 GregTwo = calendar:date_to_gregorian_days(Y2,M2,D2),
 GregTwo - GregOne.

</lang>

Output:

erlang shell:

30> c(daysbetween).
c(daysbetween).
{ok,daysbetween}
31> daysbetween:between("2019-01-01", "2019-09-30").
daysbetween:between("2019-01-01", "2019-09-30").
272

Perl 6

Dates are first class objects in Perl 6 and may have arithmetic in days done directly on them. <lang perl6>say Date.new('2019-09-30') - Date.new('2019-01-01');

say Date.new('2019-03-01') - Date.new('2019-02-01');

say Date.new('2020-03-01') - Date.new('2020-02-01');

say Date.new('2029-03-29') - Date.new('2019-03-29');

say Date.new('2019-01-01') + 90;

say Date.new('2020-01-01') + 90;

say Date.new('2019-02-29') + 30;

CATCH { default { .message.say; exit; } };</lang>

272
28
29
3653
2019-04-01
2020-03-31
Day out of range. Is: 29, should be in 1..28

Python

<lang python>

1. !/usr/bin/python

import sys

Difference between two dates = g(y2,m2,d2) - g(y1,m1,d1)

   Where g() gives us the Gregorian Calendar Day
   Inspired  by discussion at:
   https://stackoverflow.com/questions/12862226/the-implementation-of-calculating-the-number-of-days-between-2-dates

def days( y,m,d ):

  input year and month are shifted to begin the year in march
 m = (m + 9) % 12 
 y = y - m/10

  with (m*306 + 5)/10 the number of days from march 1 to the current 'm' month 
 result = 365*y + y/4 - y/100 + y/400 + (m*306 + 5)/10 + ( d - 1 )
 return result

def diff(one,two):

 [y1,m1,d1] = one.split('-')
 [y2,m2,d2] = two.split('-')
 # strings to integers
 year2 = days( int(y2),int(m2),int(d2))
 year1 = days( int(y1), int(m1), int(d1) )
 return year2 - year1

if __name__ == "__main__":

 one = sys.argv[1]
 two = sys.argv[2]
 print diff(one,two)

</lang>

Output:

python days-between.py 2019-01-01 2019-09-30
272