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Days between dates

From Rosetta Code
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.

Ada[edit]

with Ada.Calendar;
with Ada.Text_IO;
with Ada.Integer_Text_IO;
 
with GNAT.Calendar.Time_IO;
 
procedure Days_Between_Dates is
 
function Days_Between (Lower_Date  : in Ada.Calendar.Time;
Higher_Date : in Ada.Calendar.Time) return Integer
is
use Ada.Calendar;
Diff : constant Duration := Higher_Date - Lower_Date;
begin
return Integer (Diff / Day_Duration'Last);
end Days_Between;
 
procedure Put_Days_Between (Lower_Date  : in String;
Higher_Date : in String;
Comment  : in String)
is
use Ada.Text_IO;
use Ada.Integer_Text_IO;
use GNAT.Calendar.Time_IO;
 
Diff : constant Integer := Days_Between (Lower_Date => Value (Lower_Date),
Higher_Date => Value (Higher_Date));
begin
Put ("Days between " & Lower_Date & " and " & Higher_Date & " is ");
Put (Diff, Width => 5);
Put (" days -- ");
Put (Comment);
New_Line;
end Put_Days_Between;
 
begin
Put_Days_Between ("1995-11-21", "1995-11-21", "Identical dates");
Put_Days_Between ("2019-01-01", "2019-01-02", "Positive span");
Put_Days_Between ("2019-01-02", "2019-01-01", "Negative span");
Put_Days_Between ("2019-01-01", "2019-03-01", "Non-leap year");
Put_Days_Between ("2020-01-01", "2020-03-01", "Leap year");
Put_Days_Between ("1902-01-01", "1968-12-25", "Past");
Put_Days_Between ("2090-01-01", "2098-12-25", "Future");
Put_Days_Between ("1902-01-01", "2098-12-25", "Long span");
end Days_Between_Dates;
Output:
Days between 1995-11-21 and 1995-11-21 is     0 days  --  Identical dates
Days between 2019-01-01 and 2019-01-02 is     1 days  --  Positive span
Days between 2019-01-02 and 2019-01-01 is    -1 days  --  Negative span
Days between 2019-01-01 and 2019-03-01 is    59 days  --  Non-leap year
Days between 2020-01-01 and 2020-03-01 is    60 days  --  Leap year
Days between 1902-01-01 and 1968-12-25 is 24465 days  --  Past
Days between 2090-01-01 and 2098-12-25 is  3280 days  --  Future
Days between 1902-01-01 and 2098-12-25 is 71947 days  --  Long span

AppleScript[edit]

on daysBetweenDates(date1, date2)
considering numeric strings -- Allows for leading zeros having been omitted.
if (date1 = date2) then return date1 & " and " & date2 & " are the same date"
end considering
 
-- Get the components of each date.
set astid to AppleScript's text item delimiters
set AppleScript's text item delimiters to "-"
set {y1, m1, d1} to date1's text items
set {y2, m2, d2} to date2's text items
set AppleScript's text item delimiters to astid
 
-- Derive AppleScript date objects.
-- The best way to do this generally is to use the 'current date' function to obtain a date object
-- and then to set the properties of the result (or of a copy) to the required values.
-- The initial setting of the day values to 1 is to prevent overflow due to possibly
-- incompatible day and month values during the sequential setting of the properties.
-- The integer values set here are automatically coerced from the text values obtained above.
tell (current date) to set {its day, its year, its month, its day, its time, firstDate} to {1, y1, m1, d1, 0, it}
copy firstDate to secondDate
tell secondDate to set {its day, its year, its month, its day} to {1, y2, m2, d2}
 
-- Do the math.
set daysDifference to (firstDate - secondDate) div days
 
-- Format some output.
if (daysDifference > 0) then
return date1 & " comes " & daysDifference & " day" & item (((daysDifference is 1) as integer) + 1) of {"s", ""} & ¬
" after " & date2
else
return date1 & " comes " & -daysDifference & " day" & item (((daysDifference is -1) as integer) + 1) of {"s", ""} & ¬
" before " & date2
end if
end daysBetweenDates
 
return daysBetweenDates("2020-04-11", "2001-01-01") & linefeed & ¬
daysBetweenDates("2020-04-11", "2020-04-12") & linefeed & ¬
daysBetweenDates("2020-04-11", "2020-04-11")
Output:
"2020-04-11 comes 7040 days after 2001-01-01
2020-04-11 comes 1 day before 2020-04-12
2020-04-11 and 2020-04-11 are the same date"


Or, composing a function from reusable generics, and drawing on NSISO8601DateFormatter:

use AppleScript version "2.4"
use framework "Foundation"
use scripting additions
 
 
-- daysBetween :: String -> String -> Int
on daysBetween(iso8601From, iso8601To)
set midnight to "T00:00:00+00:00"
((dateFromISO8601(iso8601To & midnight) - ¬
dateFromISO8601(iso8601From & midnight)) / 86400) as integer
end daysBetween
 
 
-- dateFromISO8601 :: String -> Date
on dateFromISO8601(isoDateString)
set ca to current application
tell ca's NSISO8601DateFormatter's alloc's init()
set its formatOptions to ¬
(ca's NSISO8601DateFormatWithInternetDateTime as integer)
(its dateFromString:(isoDateString)) as date
end tell
end dateFromISO8601
 
 
---------------------------TEST----------------------------
on run
script test
on |λ|(ab)
set {a, b} to ab
set delta to daysBetween(a, b)
(a & " -> " & b & " -> " & ¬
justifyRight(5, space, delta as string)) & ¬
pluralize(delta, " day")
end |λ|
end script
 
unlines(map(test, {¬
{"2020-04-11", "2001-01-01"}, ¬
{"2020-04-11", "2020-04-12"}, ¬
{"2020-04-11", "2020-04-11"}, ¬
{"2019-01-01", "2019-09-30"}}))
end run
 
 
 
--------------------GENERIC FUNCTIONS ---------------------
 
-- Absolute value.
-- abs :: Num -> Num
on abs(x)
if 0 > x then
-x
else
x
end if
end abs
 
 
-- justifyRight :: Int -> Char -> String -> String
on justifyRight(n, cFiller, strText)
if n > length of strText then
text -n thru -1 of ((replicate(n, cFiller) as text) & strText)
else
strText
end if
end justifyRight
 
 
-- map :: (a -> b) -> [a] -> [b]
on map(f, xs)
-- The list obtained by applying f
-- to each element of xs.
tell mReturn(f)
set lng to length of xs
set lst to {}
repeat with i from 1 to lng
set end of lst to |λ|(item i of xs, i, xs)
end repeat
return lst
end tell
end map
 
 
-- mReturn :: First-class m => (a -> b) -> m (a -> b)
on mReturn(f)
-- 2nd class handler function lifted into 1st class script wrapper.
if script is class of f then
f
else
script
property |λ| : f
end script
end if
end mReturn
 
 
-- pluralize :: Int -> String -> String
on pluralize(n, s)
set m to abs(n)
if 0 = m or 2 ≤ m then
s & "s"
else
s
end if
end pluralize
 
 
-- replicate :: Int -> String -> String
on replicate(n, s)
set out to ""
if n < 1 then return out
set dbl to s
 
repeat while (n > 1)
if (n mod 2) > 0 then set out to out & dbl
set n to (n div 2)
set dbl to (dbl & dbl)
end repeat
return out & dbl
end replicate
 
 
-- unlines :: [String] -> String
on unlines(xs)
-- A single string formed by the intercalation
-- of a list of strings with the newline character.
set {dlm, my text item delimiters} to ¬
{my text item delimiters, linefeed}
set str to xs as text
set my text item delimiters to dlm
str
end unlines
Output:
2020-04-11 -> 2001-01-01 -> -7040 days
2020-04-11 -> 2020-04-12 ->     1 day
2020-04-11 -> 2020-04-11 ->     0 days
2019-01-01 -> 2019-09-30 ->   272 days

AWK[edit]

 
# syntax: GAWK -f DAYS_BETWEEN_DATES.AWK
BEGIN {
regexp = "^....-..-..$" # YYYY-MM-DD
main("1969-12-31","1970-01-01","builtin has bad POSIX start date")
main("1970-01-01","2038-01-19","builtin has bad POSIX stop date")
main("1970-01-01","2019-10-02","format OK")
main("1970-01-01","2019/10/02","format NG")
main("1995-11-21","1995-11-21","identical dates")
main("2019-01-01","2019-01-02","positive date")
main("2019-01-02","2019-01-01","negative date")
main("2019-01-01","2019-03-01","non-leap year")
main("2020-01-01","2020-03-01","leap year")
exit(0)
}
function main(date1,date2,comment, d1,d2,diff) {
printf("\t%s\n",comment)
d1 = days_builtin(date1)
d2 = days_builtin(date2)
diff = (d1 == "" || d2 == "") ? "error" : d2-d1
printf("builtin %10s to %10s = %s\n",date1,date2,diff)
d1 = days_generic(date1)
d2 = days_generic(date2)
diff = (d1 == "" || d2 == "") ? "error" : d2-d1
printf("generic %10s to %10s = %s\n",date1,date2,diff)
}
function days_builtin(ymd) { # use gawk builtin
if (ymd !~ regexp) { return("") }
if (ymd < "1970-01-01" || ymd > "2038-01-18") { return("") } # outside POSIX range
gsub(/-/," ",ymd)
return(int(mktime(sprintf("%s 0 0 0",ymd)) / (60*60*24)))
}
function days_generic(ymd, d,m,y,result) { # use Python formula
if (ymd !~ regexp) { return("") }
y = substr(ymd,1,4)
m = substr(ymd,6,2)
d = substr(ymd,9,2)
m = (m + 9) % 12
y = int(y - int(m/10))
result = 365*y + int(y/4) - int(y/100) + int(y/400) + int((m*306+5)/10) + (d-1)
return(result)
}
 
Output:
    builtin has bad POSIX start date
builtin 1969-12-31 to 1970-01-01 = error
generic 1969-12-31 to 1970-01-01 = 1
    builtin has bad POSIX stop date
builtin 1970-01-01 to 2038-01-19 = error
generic 1970-01-01 to 2038-01-19 = 24855
    format OK
builtin 1970-01-01 to 2019-10-02 = 18171
generic 1970-01-01 to 2019-10-02 = 18171
    format NG
builtin 1970-01-01 to 2019/10/02 = error
generic 1970-01-01 to 2019/10/02 = error
    identical dates
builtin 1995-11-21 to 1995-11-21 = 0
generic 1995-11-21 to 1995-11-21 = 0
    positive date
builtin 2019-01-01 to 2019-01-02 = 1
generic 2019-01-01 to 2019-01-02 = 1
    negative date
builtin 2019-01-02 to 2019-01-01 = -1
generic 2019-01-02 to 2019-01-01 = -1
    non-leap year
builtin 2019-01-01 to 2019-03-01 = 59
generic 2019-01-01 to 2019-03-01 = 59
    leap year
builtin 2020-01-01 to 2020-03-01 = 60
generic 2020-01-01 to 2020-03-01 = 60

C[edit]

 
#include<stdbool.h>
#include<string.h>
#include<stdio.h>
 
typedef struct{
int year, month, day;
}date;
 
date extractDate(char* str){
return (date){.year = 1000 * (str[0]-'0') + 100 * (str[1]-'0') + 10 * (str[2]-'0') + (str[3]-'0'), .month = 10*(str[5]-'0') + (str[6]-'0'), .day = 10*(str[8]-'0') + (str[9]-'0')};
}
 
bool isValidDate(char* str){
date newDate;
 
if(strlen(str)!=10 && str[4]!='-' && str[7]!='-'){
return false;
}
 
newDate = extractDate(str);
 
if(newDate.year<=0 || newDate.month<=0 || newDate.day<=0 || newDate.month>12 || (newDate.month==2 && newDate.day>29) ||
((newDate.month==1 || newDate.month==3 || newDate.month==5 || newDate.month==7 || newDate.month==8 || newDate.month==10 || newDate.month==12) && newDate.day>31) ||
newDate.day>30 || (newDate.year%4==0 && newDate.month==2 && newDate.month>28)){
return false;
}
 
return true;
}
 
int diffDays(date date1,date date2){
int days1, days2;
 
date1.month = (date1.month + 9)%12;
date1.year = date1.year - date1.month/10;
 
date2.month = (date2.month + 9)%12;
date2.year = date2.year - date2.month/10;
 
days1 = 365*date1.year + date1.year/4 - date1.year/100 + date1.year/400 + (date1.month*306 + 5)/10 + ( date1.day - 1 );
days2 = 365*date2.year + date2.year/4 - date2.year/100 + date2.year/400 + (date2.month*306 + 5)/10 + ( date2.day - 1 );
 
return days2 - days1;
}
 
int main(int argc,char** argv)
{
if(argc!=3){
return printf("Usage : %s <yyyy-mm-dd> <yyyy-mm-dd>",argv[0]);
}
 
if(isValidDate(argv[1])&&isValidDate(argv[2]) == false){
return printf("Dates are invalid.\n");
}
 
printf("Days Difference : %d\n", diffDays(extractDate(argv[1]),extractDate(argv[2])));
 
return 0;
}
 

Output :

[email protected]:~/doodles$ ./a.out 2019-01-01 2019-12-02
Days Difference : 335

C++[edit]

Translation of: c
#include <iomanip>
#include <iostream>
 
class Date {
private:
int year, month, day;
 
public:
Date(std::string str) {
if (isValidDate(str)) {
year = atoi(&str[0]);
month = atoi(&str[5]);
day = atoi(&str[8]);
} else {
throw std::exception("Invalid date");
}
}
 
int getYear() {
return year;
}
 
int getMonth() {
return month;
}
 
int getDay() {
return day;
}
 
// YYYY-MM-DD
static bool isValidDate(std::string str) {
if (str.length() != 10 || str[4] != '-' || str[7] != '-') {
return false;
}
 
if (!isdigit(str[0]) || !isdigit(str[1]) || !isdigit(str[2]) || !isdigit(str[3])) {
return false; // the year is not valid
}
if (!isdigit(str[5]) || !isdigit(str[6])) {
return false; // the month is not valid
}
if (!isdigit(str[8]) || !isdigit(str[9])) {
return false; // the day is not valid
}
 
int year = atoi(&str[0]);
int month = atoi(&str[5]);
int day = atoi(&str[8]);
 
// quick checks
if (year <= 0 || month <= 0 || day <= 0) {
return false;
}
if (month > 12) {
return false;
}
 
switch (month) {
case 2:
if (day > 29) {
return false;
}
if (!isLeapYear(year) && day == 29) {
return false;
}
break;
case 1:
case 3:
case 5:
case 7:
case 8:
case 10:
case 12:
if (day > 31) {
return false;
}
break;
default:
if (day > 30) {
return false;
}
break;
}
 
return true;
}
 
static bool isLeapYear(int year) {
if (year > 1582) {
return ((year % 4 == 0) && (year % 100 > 0))
|| (year % 400 == 0);
}
if (year > 10) {
return year % 4 == 0;
}
// not bothering with earlier leap years
return false;
}
 
friend std::ostream &operator<<(std::ostream &, Date &);
};
 
std::ostream &operator<<(std::ostream &os, Date &d) {
os << std::setfill('0') << std::setw(4) << d.year << '-';
os << std::setfill('0') << std::setw(2) << d.month << '-';
os << std::setfill('0') << std::setw(2) << d.day;
return os;
}
 
int diffDays(Date date1, Date date2) {
int d1m = (date1.getMonth() + 9) % 12;
int d1y = date1.getYear() - d1m / 10;
 
int d2m = (date2.getMonth() + 9) % 12;
int d2y = date2.getYear() - d2m / 10;
 
int days1 = 365 * d1y + d1y / 4 - d1y / 100 + d1y / 400 + (d1m * 306 + 5) / 10 + (date1.getDay() - 1);
int days2 = 365 * d2y + d2y / 4 - d2y / 100 + d2y / 400 + (d2m * 306 + 5) / 10 + (date2.getDay() - 1);
 
return days2 - days1;
}
 
int main() {
std::string ds1 = "2019-01-01";
std::string ds2 = "2019-12-02";
 
if (Date::isValidDate(ds1) && Date::isValidDate(ds2)) {
Date d1(ds1);
Date d2(ds2);
std::cout << "Days difference : " << diffDays(d1, d2);
} else {
std::cout << "Dates are invalid.\n";
}
 
return 0;
}
Output:
Days difference : 335

C#[edit]

using System;
using System.Globalization;
 
public class Program
{
public static void Main() => WriteLine(DateDiff("1970-01-01", "2019-10-18"));
 
public static int DateDiff(string d1, string d2) {
var a = DateTime.ParseExact(d1, "yyyy-MM-dd", CultureInfo.InvariantCulture);
var b = DateTime.ParseExact(d2, "yyyy-MM-dd", CultureInfo.InvariantCulture);
return (int)(b - a).TotalDays;
}
}
Output:
18187

COBOL[edit]

Works with: GnuCOBOL
COBOL *> days-between
*> Tectonics: cobc -xj days-between.cob
 
identification division.
program-id. days-between.
 
procedure division.
compute tally =
function integer-of-formatted-date('YYYY-MM-DD', '2019-11-24')
-
function integer-of-formatted-date('YYYY-MM-DD', '2000-01-01')
display tally
 
compute tally =
function integer-of-formatted-date('YYYYMMDD', '20191124')
-
function integer-of-formatted-date('YYYYMMDD', '20000101')
display tally
 
goback.
end program days-between.
Output:
prompt$ cobc -xj days-between-dates.cob
07267
07267

D[edit]

import std.datetime.date;
import std.stdio;
 
void main() {
auto fromDate = Date.fromISOExtString("2019-01-01");
auto toDate = Date.fromISOExtString("2019-10-07");
auto diff = toDate - fromDate;
writeln("Number of days between ", fromDate, " and ", toDate, ": ", diff.total!"days");
}
Output:
Number of days between 2019-Jan-01 and 2019-Oct-07: 279

Erlang[edit]

 
 
-module(daysbetween).
-export([between/2,dateToInts/2]).
 
% Return Year or Month or Date from datestring
dateToInts(String, POS) ->
list_to_integer( lists:nth( POS, string:tokens(String, "-") ) ).
 
% Alternative form of above
% dateToInts(String,POS) ->
% list_to_integer( lists:nth( POS, re:split(String ,"-", [{return,list},trim]) ) ).
 
% Return the number of days between dates formatted "2019-09-30"
between(DateOne,DateTwo) ->
L = [1,2,3],
[Y1,M1,D1] = [ dateToInts(DateOne,X) || X <- L],
[Y2,M2,D2] = [ dateToInts(DateTwo,X) || X <- L],
GregOne = calendar:date_to_gregorian_days(Y1,M1,D1),
GregTwo = calendar:date_to_gregorian_days(Y2,M2,D2),
GregTwo - GregOne.
 
 
 
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

Factor[edit]

Factor supports the addition and subtraction of timestamps and durations with the time+ and time- words.

USING: calendar calendar.parser kernel math prettyprint ;
 
: days-between ( ymd-str ymd-str -- n )
[ ymd>timestamp ] [email protected] time- duration>days abs ;
 
"2019-01-01" "2019-09-30" days-between .
"2016-01-01" "2016-09-30" days-between .  ! leap year
Output:
272
273

Go[edit]

package main
 
import (
"fmt"
"log"
"time"
)
 
const layout = "2006-01-02" // template for time.Parse
 
// Parameters assumed to be in YYYY-MM-DD format.
func daysBetween(date1, date2 string) int {
t1, err := time.Parse(layout, date1)
check(err)
t2, err := time.Parse(layout, date2)
check(err)
days := int(t1.Sub(t2).Hours() / 24)
if days < 0 {
days = -days
}
return days
}
 
func check(err error) {
if err != nil {
log.Fatal(err)
}
}
 
func main() {
date1, date2 := "2019-01-01", "2019-09-30"
days := daysBetween(date1, date2)
fmt.Printf("There are %d days between %s and %s\n", days, date1, date2)
 
date1, date2 = "2015-12-31", "2016-09-30"
days = daysBetween(date1, date2)
fmt.Printf("There are %d days between %s and %s\n", days, date1, date2)
}
Output:
There are 272 days between 2019-01-01 and 2019-09-30
There are 274 days between 2015-12-31 and 2016-09-30

Groovy[edit]

Translation of: Kotlin
Translation of: Java
import java.time.LocalDate
 
def fromDate = LocalDate.parse("2019-01-01")
def toDate = LocalDate.parse("2019-10-19")
def diff = fromDate - toDate
println "Number of days between ${fromDate} and ${toDate}: ${diff}"
Output:
Number of days between 2019-01-01 and 2019-10-19: 291

Haskell[edit]

import Data.Time (Day)
import Data.Time.Calendar (diffDays)
import Data.Time.Format (parseTimeM,defaultTimeLocale)
 
main = do
putStrLn $ task "2019-01-01" "2019-09-30"
putStrLn $ task "2015-12-31" "2016-09-30"
 
task :: String -> String -> String
task xs ys = "There are " ++ (show $ betweenDays xs ys) ++ " days between " ++ xs ++ " and " ++ ys ++ "."
 
betweenDays :: String -> String -> Integer
betweenDays date1 date2 = go (stringToDay date1) (stringToDay date2)
where
go (Just x) (Just y) = diffDays y x
go Nothing _ = error "Exception: Bad format first date"
go _ Nothing = error "Exception: Bad format second date"
 
stringToDay :: String -> Maybe Day
stringToDay date = parseTimeM True defaultTimeLocale "%Y-%-m-%-d" date
Output:
There are 272 days between 2019-01-01 and 2019-09-30.
There are 274 days between 2015-12-31 and 2016-09-30.

Java[edit]

Translation of: Kotlin
import java.time.LocalDate;
import java.time.temporal.ChronoUnit;
 
public class DaysBetweenDates {
public static void main(String[] args) {
LocalDate fromDate = LocalDate.parse("2019-01-01");
LocalDate toDate = LocalDate.parse("2019-10-19");
long diff = ChronoUnit.DAYS.between(fromDate, toDate);
System.out.printf("Number of days between %s and %s: %d\n", fromDate, toDate, diff);
}
}
Output:
Number of days between 2019-01-01 and 2019-10-19: 291

Julia[edit]

using Dates
 
@show Day(DateTime("2019-09-30") - DateTime("2019-01-01"))
 
@show Day(DateTime("2019-03-01") - DateTime("2019-02-01"))
 
@show Day(DateTime("2020-03-01") - DateTime("2020-02-01"))
 
@show Day(DateTime("2029-03-29") - DateTime("2019-03-29"))
 
Output:
Day(DateTime("2019-09-30") - DateTime("2019-01-01")) = 272 days
Day(DateTime("2019-03-01") - DateTime("2019-02-01")) = 28 days
Day(DateTime("2020-03-01") - DateTime("2020-02-01")) = 29 days
Day(DateTime("2029-03-29") - DateTime("2019-03-29")) = 3653 days

Kotlin[edit]

import java.time.LocalDate
import java.time.temporal.ChronoUnit
 
fun main() {
val fromDate = LocalDate.parse("2019-01-01")
val toDate = LocalDate.parse("2019-10-19")
val diff = ChronoUnit.DAYS.between(fromDate, toDate)
println("Number of days between $fromDate and $toDate: $diff")
}
Output:
Number of days between 2019-01-01 and 2019-10-19: 291

Lua[edit]

This uses os.difftime to compare two Epoch times. Not to be used with dates before 1970.

SECONDS_IN_A_DAY = 60 * 60 * 24
 
-- Convert date string as YYYY-MM-DD to Epoch time.
function parseDate (str)
local y, m, d = string.match(str, "(%d+)-(%d+)-(%d+)")
return os.time({year = y, month = m, day = d})
end
 
-- Main procedure
io.write("Enter date 1: ")
local d1 = parseDate(io.read())
io.write("Enter date 2: ")
local d2 = parseDate(io.read())
local diff = math.ceil(os.difftime(d2, d1) / SECONDS_IN_A_DAY)
print("There are " .. diff .. " days between these dates.")
Output:
Enter date 1: 1970-01-01
Enter date 2: 2019-10-02
There are 18171 days between these dates.

Perl[edit]

Would not reinvent this wheel.

use feature 'say';
use Date::Calc qw(Delta_Days);
 
say Delta_Days(2018,7,13, 2019,9,13); # triskaidekaphobia
say Delta_Days(1900,1,1, 2000,1,1); # a century
say Delta_Days(2000,1,1, 2100,1,1); # another, with one extra leap day
say Delta_Days(2020,1,1, 2019,10,1); # backwards in time
say Delta_Days(2019,2,29, 2019,3,1); # croaks
Output:
427
36524
36525
-92
Date::Calc::PP::Delta_Days(): Date::Calc::Delta_Days(): not a valid date at Days_between_dates line 10

Phix[edit]

include builtins\timedate.e 
-- specify as many or as few permitted formats as you like:
set_timedate_formats({"YYYY-MM-DD","DD/MM/YYYY","YYYY/MM/DD"})
 
constant SECONDS_TO_DAYS = 60*60*24
 
procedure test(string d1, d2, desc="")
timedate td1 = parse_date_string(d1),
td2 = parse_date_string(d2)
atom s = timedate_diff(td1,td2,DT_DAY),
d = s/SECONDS_TO_DAYS
string e = elapsed(s)&desc
printf(1,"Days between %s and %s: %d [%s]\n",{d1,d2,d,e})
end procedure
test("1969-12-31","1970-01-01")
test("1995-11-21","1995-11-21",", same date")
test("2019-01-02","2019-01-01",", negative date")
test("2019-01-01","2019-03-01",", non-leap year")
test("2020-01-01","2020-03-01",", leap year")
test("1970-01-01", "2019/10/18")
test("1970-01-01", "18/10/2019")

As shown, timedate_diff() can optionally round to the nearest whole number of days [else omit DT_DAY].
Note that elapsed() assumes all years are exactly 365 days, and in no way takes leap years into consideration (as opposed, of course, to timedate_diff() which handles them flawlessly), not that you sh/c/would ever use the string output of elapsed() in any further calculations anyway.

Output:
Days between 1969-12-31 and 1970-01-01: 1 [1 day]
Days between 1995-11-21 and 1995-11-21: 0 [0s, same date]
Days between 2019-01-02 and 2019-01-01: -1 [minus 1 day, negative date]
Days between 2019-01-01 and 2019-03-01: 59 [59 days, non-leap year]
Days between 2020-01-01 and 2020-03-01: 60 [60 days, leap year]
Days between 1970-01-01 and 2019/10/18: 18187 [49 years, 302 days]
Days between 1970-01-01 and 18/10/2019: 18187 [49 years, 302 days]

Python[edit]

 
 
#!/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)
 
Output:
python days-between.py 2019-01-01 2019-09-30
272

Raku[edit]

(formerly Perl 6) Dates are first class objects in Raku and may have arithmetic in days done directly on them.

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

REBOL[edit]

REBOL accepts a multitude of lexically recognized date! formats. date! is a builtin datatype. Slashes or dashes, year-month-day, day/month/year, or dd/monthname/year, etc. Math on dates defaults to days.

prompt$ rebol -q
>> 2019-11-24 - 2000-01-01
== 7267
>> 2019-nov-24 - 01-jan-2000
== 7267

REXX[edit]

Works with: Regina REXX

bare bones version[edit]

Programming note:   the   B   (Base)   an option for the   date   BIF which indicates to compute the number of
days since the beginning of the Gregorian calendar,   and   I   which is the option that indicates the date is in
the   ISO   (International Standards Organization standard 8601:2004)   format.

/*REXX program computes the number of days between two dates in the form of  YYYY-MM-DD */
parse arg $1 $2 . /*get 2 arguments (dates) from the C.L.*/
say abs( date('B',$1,"I") - date('B',$2,"I") ) ' days between ' $1 " and " $2
/*stick a fork in it, we're all done. */
output   when using the inputs of:     2019-10-02   2000-01-01
7214  days between  2019-10-02  and  2000-01-01

supports more variations[edit]

This REXX version supports more variations in the date format   (allows a single digit month and/or day), as well as
allowing a single asterisk   (*)   to be used for a date   (which signifies that the current date is to be used).

Commas   (,)   are inserted into numbers where appropriate.

Also, more informative error messages are generated.

/*REXX program computes the number of days between two dates in the form of  YYYY-MM-DD */
parse arg $.1 $.2 _ . 1 . . xtra /*obtain two arguments from the C.L. */
if $.1=='*' then $.1= date("I") /*obtain current date if it's an *. */
if $.2=='*' then $.2= date("I") /* " " " " " " " */
parse var $.1 yr.1 '-' mon.1 "-" dd.1 /*obtain the constituents of 1st date. */
parse var $.2 yr.2 '-' mon.2 "-" dd.2 /* " " " " 2nd " */
?.1= '1st'
?.2= '2nd'
if _ \== '' then call err "too many arguments specified: " xtra
dy.=31 /*default number of days for all months*/
parse value 30 with dy.4 1 dy.6 1 dy.9 1 dy.11 /*define 30─day months, Feb. is special*/
@notCorr= "isn't in the correct format: YYYY-MM-DD "
 
do j=1 for 2 /*examine both dates for correct format*/
if $.j =='' then call err ?.j "date was not specified."
if length(yr.j)==0 then call err ?.j "year" @notCorr '(missing)'
if isDec(yr.j) then call err ?.j "year" @notCorr '(has a non─decimal digit)'
if yr.j<1 | yr.j>9999 then call err ?.j "year" @notCorr '(not in the range 1──►9999)'
if length(mon.j)==0 then call err ?.j "month" @notCorr '(missing)'
if isDec(mon.j) then call err ?.j "month" @notCorr '(has a non─decimal digit)'
if mon.j<1 | mon.j>12 then call err ?.j "month" @notCorr '(not in the range 1──►12)'
if length(dd.j)==0 then call err ?.j "day" @notCorr '(missing)'
if isDec(dd.j) then call err ?.j "day" @notCorr '(has a non─decimal digit)'
mo= mon.j
if leapYr(yr.j) then dy.2= 29 /*Is it a leapyear? Use 29 days for Feb*/
else dy.2= 28 /*Isn't " " " 28 " " " */
if dd.j<1 | dd.j>dy.mo then call err ?.j "day" @notCorr '(day in month is invalid)'
 
yr.j= right( yr.j +0, 4, 0) /*force YYYY to be four decimal digits.*/
mon.j= right(mon.j +0, 2, 0) /* " MON " " two " " */
dd.j= right( dd.j +0, 2, 0) /* " DD " " " " " */
$.j= yr.j'-'mon.j"-"dd.j /*reconstitute a date from above parts.*/
end /*j*/
 
say commas(abs(date('B',$.1,"I") -date('B',$.2,"I"))) ' days between ' $.1 " and " $.2
exit /*stick a fork in it, we're all done. */
/*──────────────────────────────────────────────────────────────────────────────────────*/
commas: parse arg _; do c_=length(_)-3 to 1 by -3; _=insert(',', _, c_); end; return _
err: say; say '***error*** ' arg(1); exit 13 /*issue an error message (with text) */
isDec: return verify( arg(1), 1234567890) \== 0 /*insure argument is just decimal digs.*/
leapYr: arg _; ly=_//4==0; if ly==0 then return 0; ly=((_//100\==0)|_//400==0); return ly
output   when using the inputs of:     *   2000-1-1

Today   (indicated by the asterisk)   is   2019-10-2

7,214  days between  2019-10-02  and  2000-01-01

Ring[edit]

 
load "stdlib.ring"
 
DaysBetween = [["1995-11-21","1995-11-21"],
["2019-01-01","2019-01-02"],
["2019-01-02","2019-01-01"],
["2019-01-01","2019-03-01"],
["2020-01-01","2020-03-01"],
["1902-01-01","1968-12-25"],
["2090-01-01","2098-12-25"]]
 
for n = 1 to len(DaysBetween)
date1 = DaysBetween[n][1]
date2 = DaysBetween[n][2]
date3 = substr(date1,9,2) + "/" + substr(date1,6,2) + "/" + substr(date1,1,4)
date4 = substr(date2,9,2) + "/" + substr(date2,6,2) + "/" + substr(date2,1,4)
 ? "Days between " + DaysBetween[n][1] + " and " + DaysBetween[n][2] + ": " + diffdays(date4,date3)
next
 
Output:
Days between 1995-11-21 and 1995-11-21: 0
Days between 2019-01-01 and 2019-01-02: 1
Days between 2019-01-02 and 2019-01-01: -1
Days between 2019-01-01 and 2019-03-01: 59
Days between 2020-01-01 and 2020-03-01: 60
Days between 1902-01-01 and 1968-12-25: 0
Days between 2090-01-01 and 2098-12-25: 3280

Ruby[edit]

require "date"
 
d1, d2 = Date.parse("2019-1-1"), Date.parse("2019-10-19")
 
p (d1 - d2).to_i # => -291
p (d2 - d1).to_i # => 291
 

Scala[edit]

object DaysBetweenDates {
 
/*Inspired by the Python version of the algorithm and the discussion here
https://stackoverflow.com/questions/12862226/the-implementation-of-calculating-the-number-of-days-between-2-dates.*/

 
/**Transform a date into a day number in the Gregorian Calendar*/
def dateToDays(year : Int, month : Int, day : Int ) : Int = {
val m = (month+ 9) % 12
val y = year - m/10
val d = day
365*y + y/4 - y/100 + y/400 + (m*306 + 5)/10 + (d - 1)
}
 
/**Compute the difference of days between both input dates*/
def daysDifference(firstDate : String, secondDate : String) : Int = {
val firstDateTuple = firstDate.split('-') match { case Array(a, b, c) => (a, b, c) }
val secondDateTuple = secondDate.split('-') match { case Array(a, b, c) => (a, b, c) }
 
val firstYear = dateToDays( firstDateTuple._1.toInt, firstDateTuple._2.toInt, firstDateTuple._3.toInt)
val secondYear = dateToDays( secondDateTuple._1.toInt, secondDateTuple._2.toInt, secondDateTuple._3.toInt )
 
return secondYear - firstYear
}
 
def main(args: Array[String]): Unit = {
println(daysDifference("2019-01-01", "2019-09-30"))
println(daysDifference("1995-11-21", "1995-11-21"))
println(daysDifference("2019-01-01", "2019-01-02"))
println(daysDifference("2019-01-02", "2019-01-01"))
println(daysDifference("2019-01-01", "2019-03-01"))
println(daysDifference("2020-01-01", "2020-03-01"))
println(daysDifference("1902-01-01", "1968-12-25"))
println(daysDifference("2090-01-01", "2098-12-25"))
println(daysDifference("1902-01-01", "2098-12-25"))
}
 
}
 
Output:
272
0
1
-1
59
60
24465
3280
71947

Sidef[edit]

require('Date::Calc')
 
func days_diff(a,b) {
%S<Date::Calc>.Delta_Days(a.split('-')..., b.split('-')...)
}
 
var date1 = "1970-01-01"
var date2 = "2019-10-02"
 
say "Date 1: #{date1}"
say "Date 2: #{date2}"
 
var days = days_diff(date1, date2)
 
say "There are #{days} days between these dates"
Output:
Date 1: 1970-01-01
Date 2: 2019-10-02
There are 18171 days between these dates

Swift[edit]

import Foundation
 
func daysFromTimeInterval(_ interval: Double) -> Int {
return Int(interval) / 86400
}
 
let formatter = DateFormatter()
 
formatter.dateFormat = "yyyy-MM-dd"
 
print("Enter date one (yyyy-MM-dd): ", terminator: "")
 
guard let date1Str = readLine(strippingNewline: true), let date1 = formatter.date(from: date1Str) else {
fatalError("Invalid date two")
}
 
print("Enter date two (yyyy-MM-dd): ", terminator: "")
 
guard let date2Str = readLine(strippingNewline: true), let date2 = formatter.date(from: date2Str) else {
fatalError("Invalid date two")
}
 
let (start, end) = date1 > date2 ? (date2, date1) : (date1, date2)
let days = daysFromTimeInterval(DateInterval(start: start, end: end).duration)
 
print("There are \(days) days between \(start) and \(end)")
Output:
Enter date one (yyyy-MM-dd): 2019-01-01
Enter date two (yyyy-MM-dd): 2019-12-02
There are 335 days between 2019-01-01 05:00:00 +0000 and 2019-12-02 05:00:00 +0000

UNIX Shell[edit]

Works with: Bourne Again Shell
# return true if year is leap in Gregorian calendar
leap() {
local -i year
year=$1
if (( year % 4 )); then return 1; fi
if (( year % 100 )); then return 0; fi
! (( year % 400 ))
}
 
# convert date to Gregorian day count (Rata Die), where RD 1 = January 1, 1 CE
rd() {
local year month day
IFS=- read year month day <<<"$1"
local -i elapsed_years=year-1
(( day += elapsed_years * 365 ))
(( day += elapsed_years/4 ))
(( day -= elapsed_years/100 ))
(( day += elapsed_years/400 ))
local month_lengths=(31 28 31 30 31 30 31 31 30 31 30 31)
if leap "$year"; then let month_lengths[1]+=1; fi
local m
for (( m=0; m<month-1; ++m)); do
(( day += month_lengths[m] ))
done
printf '%d\n' "$day"
}
 
days_between() {
local -i date1 date2
date1=$(rd "$1")
date2=$(rd "$2")
printf '%d\n' $(( date2 - date1 ))
}
 
days_between 1970-01-01 2019-12-04
 
Output:
18234

Visual Basic .NET[edit]

Translation of: C#
Imports System.Globalization
 
Module Module1
 
Function DateDiff(d1 As String, d2 As String) As Integer
Dim a = DateTime.ParseExact(d1, "yyyy-MM-dd", CultureInfo.InvariantCulture)
Dim b = DateTime.ParseExact(d2, "yyyy-MM-dd", CultureInfo.InvariantCulture)
Return (b - a).TotalDays
End Function
 
Sub Main()
Console.WriteLine(DateDiff("1970-01-01", "2019-10-18"))
End Sub
 
End Module
Output:
18187

Wren[edit]

Library: Wren-date
import "/date" for Date
 
var datePairs = [
["1995-11-21", "1995-11-21"],
["2019-01-01", "2019-01-02"],
["2019-01-02", "2019-01-01"],
["2019-01-01", "2019-03-01"],
["2020-01-01", "2020-03-01"],
["1902-01-01", "1968-12-25"],
["2090-01-01", "2098-12-25"],
["1902-01-01", "2098-12-25"],
["1970-01-01", "2019-10-18"],
["2019-03-29", "2029-03-29"],
["2020-02-29", "2020-03-01"]
]
for (dates in datePairs) {
var date1 = Date.parse(dates[0])
var date2 = Date.parse(dates[1])
var days = (date2 - date1).days
Date.default = Date.isoDate
System.print("Days between %(date1) and %(date2) = %(days)")
}
Output:
Days between 1995-11-21 and 1995-11-21 = 0
Days between 2019-01-01 and 2019-01-02 = 1
Days between 2019-01-02 and 2019-01-01 = -1
Days between 2019-01-01 and 2019-03-01 = 59
Days between 2020-01-01 and 2020-03-01 = 60
Days between 1902-01-01 and 1968-12-25 = 24465
Days between 2090-01-01 and 2098-12-25 = 3280
Days between 1902-01-01 and 2098-12-25 = 71947
Days between 1970-01-01 and 2019-10-18 = 18187
Days between 2019-03-29 and 2029-03-29 = 3653
Days between 2020-02-29 and 2020-03-01 = 1

zkl[edit]

var [const] TD=Time.Date;
today:=TD.parseDate("--"); // "yyyy-mm-dd" and variations --> (y,m,d)
// or Time.Clock.UTC --> (y,m,d,h,m,s)
then:=TD.parseDate("2018-9-30");
diff:=TD.deltaDays(then,today.xplode()); // ( (y,m,d), y,m,d )
println("Number of days between %s and %s: %d".fmt(then,today,diff));
println("Number of days between %s and %s: %d".fmt(
TD.toYMDString(today.xplode()), // to(y,m,d) not to((y,m,d))
TD.toYMDString(then.xplode()),
TD.deltaDays(today,then.xplode())));
Output:
Number of days between L(2018,9,30) and L(2019,9,30): 365
Number of days between 2019-09-30 and 2018-09-30: -365