# Find number of days between two given dates

Given two dates, find total number of days between them. The count of days must be calculated in O(1) time and O(1) auxiliary space.

Examples:

```Input: dt1 = {10, 2, 2014}
dt2 = {10, 3, 2015}
Output: 393
dt1 represents "10-Feb-2014" and dt2 represents "10-Mar-2015"
The difference is 365 + 28

Input: dt1 = {10, 2, 2000}
dt2 = {10, 3, 2000}
Output: 29
Note that 2000 is a leap year

Input: dt1 = {10, 2, 2000}
dt2 = {10, 2, 2000}
Output: 0
Both dates are same

Input:   dt1 = {1, 2, 2000};
dt2 = {1, 2, 2004};
Output: 1461
Number of days is 365*4 + 1```

## We strongly recommend that you click here and practice it, before moving on to the solution.

One Naive Solution is to start from dt1 and keep counting days till dt2 is reached. This solution requires more than O(1) time.
A Better and Simple solution is to count total number of days before dt1 from i.e., total days from 00/00/0000 to dt1, then count total number of days before dt2. Finally return the difference between two counts.

```Let the given two dates be "1-Feb-2000" and "1-Feb-2004"
dt1 = {1, 2, 2000};
dt2 = {1, 2, 2004};

Count number of days before dt1. Let this count be n1.
Every leap year adds one extra day (29 Feb) to total days.

n1 = 2000*365 + 31 + 1 + Number of leap years

Count of leap years for a date 'd/m/y' can be calculated
using following formula:
Number leap years
= floor(y/4) - floor(y/100) + floor(y/400) if m > 2
= floor((y-1)/4) - floor((y-1)/100) + floor((y-1)/400) if m <= 2
All above divisions must be done using integer arithmetic
so that the remainder is ignored.

For 01/01/2000, leap year count is 1999/4 - 1999/100
+ 1999/400 which is 499 - 19 + 4 = 484
Therefore n1 is 2000*365 + 31 + 1 + 484

Similarly, count number of days before dt2. Let this
count be n2.

Finally return n2-n1
```

Below is the implementation of above idea.

## C++

 `// C++ program two find number of days between two given dates ` `#include ` `using` `namespace` `std; ` ` `  `// A date has day 'd', month 'm' and year 'y' ` `struct` `Date ` `{ ` `    ``int` `d, m, y; ` `}; ` ` `  `// To store number of days in all months from January to Dec. ` `const` `int` `monthDays = {31, 28, 31, 30, 31, 30, ` `                           ``31, 31, 30, 31, 30, 31}; ` ` `  `// This function counts number of leap years before the ` `// given date ` `int` `countLeapYears(Date d) ` `{ ` `    ``int` `years = d.y; ` ` `  `    ``// Check if the current year needs to be considered ` `    ``// for the count of leap years or not ` `    ``if` `(d.m <= 2) ` `        ``years--; ` ` `  `    ``// An year is a leap year if it is a multiple of 4, ` `    ``// multiple of 400 and not a multiple of 100. ` `    ``return` `years / 4 - years / 100 + years / 400; ` `} ` ` `  `// This function returns number of days between two given ` `// dates ` `int` `getDifference(Date dt1, Date dt2) ` `{ ` `    ``// COUNT TOTAL NUMBER OF DAYS BEFORE FIRST DATE 'dt1' ` ` `  `    ``// initialize count using years and day ` `    ``long` `int` `n1 = dt1.y*365 + dt1.d; ` ` `  `    ``// Add days for months in given date ` `    ``for` `(``int` `i=0; i

## Java

 `// Java program two find number of  ` `// days between two given dates ` ` `  `class` `GFG  ` `{ ` ` `  `    ``// A date has day 'd', month 'm' and year 'y' ` `    ``static` `class` `Date  ` `    ``{ ` `        ``int` `d, m, y; ` ` `  `        ``public` `Date(``int` `d, ``int` `m, ``int` `y) ` `        ``{ ` `            ``this``.d = d; ` `            ``this``.m = m; ` `            ``this``.y = y; ` `        ``} ` ` `  `    ``}; ` ` `  `    ``// To store number of days in  ` `    ``// all months from January to Dec. ` `    ``static` `int` `monthDays[] = {``31``, ``28``, ``31``, ``30``, ``31``, ``30``, ` `                            ``31``, ``31``, ``30``, ``31``, ``30``, ``31``}; ` ` `  `    ``// This function counts number of  ` `    ``// leap years before the given date ` `    ``static` `int` `countLeapYears(Date d)  ` `    ``{ ` `        ``int` `years = d.y; ` ` `  `        ``// Check if the current year needs to be considered ` `        ``// for the count of leap years or not ` `        ``if` `(d.m <= ``2``)  ` `        ``{ ` `            ``years--; ` `        ``} ` ` `  `        ``// An year is a leap year if it is a multiple of 4, ` `        ``// multiple of 400 and not a multiple of 100. ` `        ``return` `years / ``4` `- years / ``100` `+ years / ``400``; ` `    ``} ` ` `  `    ``// This function returns number  ` `    ``// of days between two given dates ` `    ``static` `int` `getDifference(Date dt1, Date dt2) ` `    ``{ ` `        ``// COUNT TOTAL NUMBER OF DAYS BEFORE FIRST DATE 'dt1' ` ` `  `        ``// initialize count using years and day ` `        ``int` `n1 = dt1.y * ``365` `+ dt1.d; ` ` `  `        ``// Add days for months in given date ` `        ``for` `(``int` `i = ``0``; i < dt1.m - ``1``; i++)  ` `        ``{ ` `            ``n1 += monthDays[i]; ` `        ``} ` ` `  `        ``// Since every leap year is of 366 days, ` `        ``// Add a day for every leap year ` `        ``n1 += countLeapYears(dt1); ` ` `  `        ``// SIMILARLY, COUNT TOTAL NUMBER OF DAYS BEFORE 'dt2' ` `        ``int` `n2 = dt2.y * ``365` `+ dt2.d; ` `        ``for` `(``int` `i = ``0``; i < dt2.m - ``1``; i++) ` `        ``{ ` `            ``n2 += monthDays[i]; ` `        ``} ` `        ``n2 += countLeapYears(dt2); ` ` `  `        ``// return difference between two counts ` `        ``return` `(n2 - n1); ` `    ``} ` ` `  `    ``// Driver code ` `    ``public` `static` `void` `main(String[] args)  ` `    ``{ ` `        ``Date dt1 = ``new` `Date(``1``, ``2``, ``2000``); ` `        ``Date dt2 = ``new` `Date(``1``, ``2``, ``2004``); ` `        ``System.out.println(``"Difference between two dates is "` `+  ` `                            ``getDifference(dt1, dt2)); ` `    ``} ` `}  ` ` `  `// This code is contributed by 29AjayKumar `

## Python3

 `# Python3 program two find number of  ` `# days between two given dates  ` ` `  `# A date has day 'd', month  ` `# 'm' and year 'y' ` `class` `Date:  ` `    ``def` `__init__(``self``, d, m, y):  ` `        ``self``.d ``=` `d          ` `        ``self``.m ``=` `m  ` `        ``self``.y ``=` `y  ` ` `  `# To store number of days in ` `# all months from January to Dec.  ` `monthDays ``=` `[``31``, ``28``, ``31``, ``30``, ``31``, ``30``,  ` `             ``31``, ``31``, ``30``, ``31``, ``30``, ``31` `] ` ` `  `# This function counts number of  ` `# leap years before the given date  ` `def` `countLeapYears(d): ` `     `  `    ``years ``=` `d.y  ` `     `  `    ``# Check if the current year needs  ` `    ``# to be considered for the count ` `    ``# of leap years or not  ` `    ``if` `(d.m <``=` `2``) : ` `        ``years ``-``=` `1` `         `  `    ``# An year is a leap year if it is a ` `    ``# multiple of 4, multiple of 400 and ` `    ``# not a multiple of 100.  ` `    ``return` `int``(years ``/` `4` `-` `years ``/` `100` `+`  `                           ``years ``/` `400` `) ` ` `  `# This function returns number of  ` `# days between two given dates  ` `def` `getDifference(dt1, dt2) : ` ` `  `    ``# COUNT TOTAL NUMBER OF DAYS  ` `    ``# BEFORE FIRST DATE 'dt1'  ` ` `  `    ``# initialize count using years and day  ` `    ``n1 ``=` `dt1.y ``*` `365` `+` `dt1.d  ` ` `  `    ``# Add days for months in given date  ` `    ``for` `i ``in` `range``(``0``, dt1.m ``-` `1``) : ` `        ``n1 ``+``=` `monthDays[i]  ` ` `  `    ``# Since every leap year is of 366 days,  ` `    ``# Add a day for every leap year  ` `    ``n1 ``+``=` `countLeapYears(dt1)  ` ` `  `    ``# SIMILARLY, COUNT TOTAL NUMBER  ` `    ``# OF DAYS BEFORE 'dt2'  ` `    ``n2 ``=` `dt2.y ``*` `365` `+` `dt2.d  ` `    ``for` `i ``in` `range``(``0``, dt2.m ``-` `1``) : ` `        ``n2 ``+``=` `monthDays[i]  ` `    ``n2 ``+``=` `countLeapYears(dt2)  ` ` `  `    ``# return difference between  ` `    ``# two counts  ` `    ``return` `(n2 ``-` `n1)  ` ` `  `# Driver Code ` `dt1 ``=` `Date(``1``, ``2``, ``2000` `) ` `dt2 ``=` `Date(``1``, ``2``, ``2004` `) ` ` `  `print``(``"Difference between two dates is"``,  ` `                ``getDifference(dt1, dt2)) ` ` `  `# This code is contributed by Smitha `

## C#

 `// C# program two find number of  ` `// days between two given dates ` `using` `System; ` ` `  `class` `GFG  ` `{ ` ` `  `    ``// A date has day 'd', month 'm' and year 'y' ` `    ``public` `class` `Date  ` `    ``{ ` `        ``public` `int` `d, m, y; ` ` `  `        ``public` `Date(``int` `d, ``int` `m, ``int` `y) ` `        ``{ ` `            ``this``.d = d; ` `            ``this``.m = m; ` `            ``this``.y = y; ` `        ``} ` ` `  `    ``}; ` ` `  `    ``// To store number of days in  ` `    ``// all months from January to Dec. ` `    ``static` `int` `[]monthDays = {31, 28, 31, 30, 31, 30, ` `                            ``31, 31, 30, 31, 30, 31}; ` ` `  `    ``// This function counts number of  ` `    ``// leap years before the given date ` `    ``static` `int` `countLeapYears(Date d)  ` `    ``{ ` `        ``int` `years = d.y; ` ` `  `        ``// Check if the current year  ` `        ``// needs to be considered ` `        ``// for the count of leap years or not ` `        ``if` `(d.m <= 2)  ` `        ``{ ` `            ``years--; ` `        ``} ` ` `  `        ``// An year is a leap year if it is  ` `        ``// a multiple of 4, multiple of 400 ` `        ``// and not a multiple of 100. ` `        ``return` `years / 4 - years / 100 + years / 400; ` `    ``} ` ` `  `    ``// This function returns number  ` `    ``// of days between two given dates ` `    ``static` `int` `getDifference(Date dt1, Date dt2) ` `    ``{ ` `        ``// COUNT TOTAL NUMBER OF DAYS  ` `        ``// BEFORE FIRST DATE 'dt1' ` ` `  `        ``// initialize count using years and day ` `        ``int` `n1 = dt1.y * 365 + dt1.d; ` ` `  `        ``// Add days for months in given date ` `        ``for` `(``int` `i = 0; i < dt1.m - 1; i++)  ` `        ``{ ` `            ``n1 += monthDays[i]; ` `        ``} ` ` `  `        ``// Since every leap year is of 366 days, ` `        ``// Add a day for every leap year ` `        ``n1 += countLeapYears(dt1); ` ` `  `        ``// SIMILARLY, COUNT TOTAL  ` `        ``// NUMBER OF DAYS BEFORE 'dt2' ` `        ``int` `n2 = dt2.y * 365 + dt2.d; ` `        ``for` `(``int` `i = 0; i < dt2.m - 1; i++) ` `        ``{ ` `            ``n2 += monthDays[i]; ` `        ``} ` `        ``n2 += countLeapYears(dt2); ` ` `  `        ``// return difference between two counts ` `        ``return` `(n2 - n1); ` `    ``} ` ` `  `    ``// Driver code ` `    ``public` `static` `void` `Main(String[] args)  ` `    ``{ ` `        ``Date dt1 = ``new` `Date(1, 2, 2000); ` `        ``Date dt2 = ``new` `Date(1, 2, 2004); ` `        ``Console.WriteLine(``"Difference between two dates is "` `+  ` `                            ``getDifference(dt1, dt2)); ` `    ``} ` `}  ` ` `  `// This code is contributed by PrinciRaj1992 `

Output:

`Difference between two dates is 1461`

Note : The above program follows the Gregorian Calender from the beginning of the time.

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