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

Days between dates
You are encouraged to solve this task according to the task description, using any language you may know.

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.

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

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