Number of days between dates

[Varun Gokulnath]

Hi,
I'm trying to write a program that computes the number of days between two dates. Can anyone suggest how i can achieve this. I'm quite sure I'm supposed to use the Calendar class. Thanks in advance.

[Seetharaman Venkatasamy]

something like:

long diff = endDate.getTime() – startDate.getTime();//in Milli seconds
int numOfDays = diff/(1000*60*60*24);

[Alexey Dubinin]

This is bad solution because daylight saving changes can lead to error in calculations. You can try Joda time library.

[Varun Gokulnath]

Thanks Seetharaman but I don't think the "-" operator can be applied to two Date object operands.

[Rob Spoor]

Except that doesn't work well with daylight savings time.

In pseudo code, assuming first <= second:

int count = 0;
Calendar temp = first.clone();
while (temp < second) // only check for days
{
    increment temp by 1 day
    count++;
}

A bit more advanced:

int count = (day of year of second) - (day of year of first);
Calendar temp = first.clone();
while (year of temp < year of second)
{
    count += (number of days in year of temp)
    increment temp by 1 year
}

This latter can be explained with two examples:

1) February 1st 2010 and March 1st 2011.
count = (31 + 28 + 1) - (31 + 1) = 28.
2010 < 2011 so count += 365 => count = 393.

2) March 1st 2010 and February 1st 2011.
count = (31 + 1) - (31 + 28 + 1) = -28 (yes, negative; that'll be corrected).
2010 < 2011 so count += 365 => count = 337.


The methods to use from Calendar are add for adding a day / year, and getActualMaximum to get the numbers in the year. getActualMaximum will return 365 or 366, getMaximum will always return 366.

[Seetharaman Venkatasamy]

Alexey Dubinin wrote:This is bad solution because daylight saving changes can lead to error in calculations.

thanks for remembering the daylight saving problem .

Varun Gokulnath wrote:Thanks Seetharaman but I don't think the "-" operator can be applied to two Date object operands.

getTime returns long

[Varun Gokulnath]

Thanks Alexey, Seetharaman and Rob.
@Seetharaman : I am able to get a good estimate by using your method

import java.util.*;
class DateDiff
{
	public static void main(String [] args)
	{
		Calendar c1=Calendar.getInstance();
		c1.set(2011,5, 29 );
		Calendar c2=Calendar.getInstance();
		c2.set(2012,5,30);

		Date d1=c1.getTime();
		Date d2=c2.getTime();

		long diff=d2.getTime()-d1.getTime();
        int noofdays=(int)(diff/(1000*24*60*60));
		System.out.println(noofdays);
	}
}

@Rob:Can you tell me if I am using your method correctly because I am not arriving at the right answer.

 int count = 0;   
		Calendar temp = (Calendar)c1.clone();   
		while ((int)(temp.getTime().getTime()) < (int)(c2.getTime().getTime())) // only check for days   
		{   
		    temp.add(Calendar.DATE, 1); 
		    count++;   
		}  
		System.out.println(count);

[Rob Spoor]

Varun Gokulnath wrote:

while ((int)(temp.getTime().getTime()) < (int)(c2.getTime().getTime())) // only check for days   

That doesn't compare the days; that compares the time in milliseconds after casting it to int. No idea why you did that cast anyway. Without it you should be very close, except if c1's hour would be one larger than c2's hour then you would get one day less.

Anyway, to compare if two dates are the same date you need to check two or three fields:
1) are the year, the month and the day of month the same.
2) are the year and day of year the same.

if (isSameDay(temp, c2))

private boolean isSameDay(Calendar c1, Calendar c2)
{
    return c1.get(Calendar.YEAR) == c2.get(Calendar.YEAR) && c1.get(Calendar.DAY_OF_YEAR) == c2.get(Calendar.OF_YEAR);
}