Number of days between dates

[Gvarun Rao]

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.

[Gvarun Rao]

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

[Gvarun Rao]

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);
}

[kevinj canlas]

how about in using a jcalendar




how to compute the number of days between two dates by using datechooser in java

[Maneesh Godbole]

You were already provided an anwer here right? So what part are you having trouble with? Post your SSCCE code so we can help point you in the right direction

[Winston Gutkowski]

Varun Gokulnath wrote: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.

Are you "quite sure [you're] supposed to use the Calendar class" because you think you need to, or because you were told to?

There's a big difference, because you DON'T need a Calendar to compute the number of days between two dates - at least not if they're java.util.Date's - and actually it probably muddies the waters.

Furthermore, you don't even need to "estimate" it, viz from your own code:

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();

   System.out.println(diffDays(d1, d2));

   int wholeDays = (int) diffDays(d1, d2);
   System.out.println(wholeDays);
}

private static final double DAY_MILLIS = 1000.0 * 24.0 * 60.0 * 60.0;

public static final double diffDays(Date from, Date to) {
   return (to.getTime() - from.getTime()) / DAY_MILLIS;
}

The method returns an extremely accurate computation of the number of days difference, which you can then truncate or round or convert to a smaller unit any way you see fit.

NOTE: The second thing that has been discussed here (seeing whether two dates are on the same day) is something completely different, because that is a comparison of WALL-CLOCK times - and for that, you DO need a Calendar ... and possibly an explicit timezone.

HIH

Winston

[James Boswell]

Varun

What stage are you at with your code? Could you show us what you have done so far? Winston makes a good point regarding Calendar. Is the starting point for your work dates which are entered by a user / read from some data source or does the code simply have to calculate the difference in days between 2 random dates?

[Stuart A. Burkett]

Winston, James
Before you get too involved with Varun's problem, check the date of his original post.

[Winston Gutkowski]

Stuart A. Burkett wrote:Before you get too involved with Varun's problem, check the date of his original post.

Dang! Always catches me out. Don't I feel the idiot....

Winston

[James Boswell]

Excellent! Lets work out the number of days between the original post and now

[Ryan Bishop]

I'm working on trying to calculate the difference between today and the first day of summer (June 21) for this year and the difference for next year.
I've tried several things and just don't understand the calendar class.

The first way I did was this:

import java.util.*;

public class Summer
{
 public static void main(String[] args)
 {
   GregorianCalendar current = new GregorianCalendar();
   GregorianCalendar firstSummer = new GregorianCalendar(2014, 5, 21);
   GregorianCalendar endYear = new GregorianCalendar(2014, 11, 31);
   GregorianCalendar secondSummer = new GregorianCalendar(2015, 5, 21);
   
   long diff1 = firstSummer.getTime() - current.getTime();
   
   System.out.println("Summer is in " + diff1 + " days"); 
  }
}

This returns an error in the long diff1 where I can't subtract a date from a date.

The other thing I tried was this:

import java.util.*;

public class Summer
{
 public static void main(String[] args)
 {
   Date current = new Date();
   Date firstSummer = new Date(2014, 5, 21);
   Date endYear = new Date(2014, 11, 31);
   Date secondSummer = new Date(2015, 5, 21);
   
   long diff1 = firstSummer.getTime() - current.getTime();
   
   System.out.println("Summer is in " + diff1 + " days");    
 }
}

This gives me the errors below but if I ignore those and run it the answer comes out to be 59967771772956 days, which is clearly off by a day or two:) I haven't even tried trying to calculate for next summer. Am I missing something basic, like somehow converting the date to an integer?

Note: Summer.java uses or overrides a deprecated API.
Note: Recompile with -Xlint:deprecation for details.

[Campbell Ritchie]

It means not to use the no‑arguments Date() constructor. Look at its API to find out what they think you should use instead.

[Ryan Bishop]

It says to use GregorianCalendar, so I'm back at the first issue.

[Campbell Ritchie]

If you use getTime, you get the duration since 1st January 1970 in milliseconds (I think), as a long. You can see that sort of thing from the older posts.
You can get Date objects from the Calendar objects; again look at the old posts. That gets you out of having to use the deprecated constructor.

[Ryan Bishop]

I can't thank you enough for all the help you guys have given me. I redid it like below and it works. I'm not sure
if it's the best way but at least I don't want to throw my computer out the window right this second:)

import java.util.*;  
      
public class Summer  
{  
  public static void main(String[] args)  
  {  
    
   Calendar current = Calendar.getInstance();  
   Calendar firstSummer = Calendar.getInstance();
   firstSummer.set(2014, 5, 21);  
   Calendar endYear = Calendar.getInstance();
   endYear.set(2014, 11, 31);  
   Calendar secondSummer = Calendar.getInstance();
   secondSummer.set(2015, 5, 21); 
   
   Date d1=current.getTime();  
   Date d2=firstSummer.getTime(); 
   Date d3=endYear.getTime();
   Date d4=secondSummer.getTime();
 
     
   long diff = d2.getTime() - d1.getTime();  
     
   System.out.println("Summer is in " + diff/(1000*24*60*60) + " days");   
  }             
}  

[Campbell Ritchie]

Don't use 5 or 11; use Calendar.JUNE, Calendar.DECEMBER, etc. Remember that technique rounds down the number of days; from 11.59pm Saturday to 11.58pm and 59″ Sunday counts as 0 days.

And you're welcome