Difference between two dates: a complete solution

Question

There have been many posts asking how to get the number of days between two dates. There is no method in java like the VB method dateDiff(), and there is only the method date.getTime() which gives you the number of milliseconds since Jan 1 1970, the Epoch. I have not seen a solution that makes use of the fact that there are 86400000 milliseconds per day, only when trying to get the number of days , i.e. multiples of 24 hours between two millisecond points in time.
So what if date1 was Oct 26th 11.30pm and date2 was Oct 27th 12.30am? If you get the millisecond difference, and divide by 86400000, you do not notice that these two times are on separate days, but only one hour apart.
My solution takes account of the modulo (%) function. If you use the date.getTime() method, and use % 86400000 on this number of milliseconds, it will tell you how far (milliseconds) this date is into this day, after 12am. If you subtract this amount from the total number of milliseconds, then divide by 86400000, this will give you the number of days after the Epoch, so you are able to know which day the millisecond time is in. If you want to know the days between two millisecond times, this will tell you, whereas just dividing by 86400000, will not, and may be misleading.
I think this could be incorporated into the Date class easily. There are some problems though, as not every day has 86400000 milliseconds, if you are talking about days when Daylight Saying Time begins or ends. Doing calculations like that will create errors, unless you know whether each date is in Daylight Saving Time or not. Leap years are OK, unless want to work out the number of years from the number of days by dividing by 365, as calculating numbers of days is only done by looking at a 24 hour time period. However where 'leap seconds' have been added, like once every ten years, there may be small errors. This was not included in anyone elses calculations.
So the calculation would go like this:
public class BetterDate extends Date
public int dayDifference(Date d)
long time = this.getTime() ; // gets milliseconds
long timed = d.getTime() ; // gets milliseconds of other date
int days = (time - (time % 86400000))/86400000 ; // number of days
int daysd = (timed - (timed % 86400000))/86400000 ; // number of days of other date
int difference = daysd - days ; // difference in days between two dates
return difference ;
If you want you can use the value (time % 86400000), and modulo this amount with the number of milliseconds in one hour, then subtract the result from the (time % 86400000) and divide by the millliseconds in one hour to give you the number of hours, but there are JDK functions that you can use to get the hour if needed.
This method is not perfect, but for most cases will get you the difference in days between two dates.
James

Answer 1

Hi James,
I tried out your solution, but it seems to fail on dates without times. I tries the difference between 31/03/2002 and 01/04/2002 and the result is 0. I guess it is because the times are assumed to be 00:00 as no times are mentioned and 31st March I believe is the date when DST change will take place this year. If I set the two times as well to a time much later than midnight for example 0600 then I seem to get the right answer.
Joag
There have been many posts asking how to get the
number of days between two dates. There is no method
in java like the VB method dateDiff(), and there is
only the method date.getTime() which gives you the
number of milliseconds since Jan 1 1970, the Epoch. I
have not seen a solution that makes use of the fact
that there are 86400000 milliseconds per day, only
when trying to get the number of days , i.e. multiples
of 24 hours between two millisecond points in time.
So what if date1 was Oct 26th 11.30pm and date2 was
Oct 27th 12.30am? If you get the millisecond
difference, and divide by 86400000, you do not notice
that these two times are on separate days, but only
one hour apart.
My solution takes account of the modulo (%) function.
If you use the date.getTime() method, and use %
86400000 on this number of milliseconds, it will tell
you how far (milliseconds) this date is into this day,
after 12am. If you subtract this amount from the total
number of milliseconds, then divide by 86400000, this
will give you the number of days after the Epoch, so
you are able to know which day the millisecond time is
in. If you want to know the days between two
millisecond times, this will tell you, whereas just
dividing by 86400000, will not, and may be
misleading.
I think this could be incorporated into the Date class
easily. There are some problems though, as not every
day has 86400000 milliseconds, if you are talking
about days when Daylight Saying Time begins or ends.
Doing calculations like that will create errors,
unless you know whether each date is in Daylight
Saving Time or not. Leap years are OK, unless want to
work out the number of years from the number of days
by dividing by 365, as calculating numbers of days is
only done by looking at a 24 hour time period. However
where 'leap seconds' have been added, like once every
ten years, there may be small errors. This was not
included in anyone elses calculations.
So the calculation would go like this:
public class BetterDate extends Date
public int dayDifference(Date d)
long time = this.getTime() ; // gets milliseconds
long timed = d.getTime() ; // gets milliseconds of
other date
int days = (time - (time % 86400000))/86400000 ; //
number of days
int daysd = (timed - (timed % 86400000))/86400000 ; //
number of days of other date
int difference = daysd - days ; // difference in days
between two dates
return difference ;
If you want you can use the value (time % 86400000),
and modulo this amount with the number of milliseconds
in one hour, then subtract the result from the (time %
86400000) and divide by the millliseconds in one hour
to give you the number of hours, but there are JDK
functions that you can use to get the hour if needed.
This method is not perfect, but for most cases will
get you the difference in days between two dates.
James