Sum two dates in Java

If you are using the Date object, you can just do:

Date d1 = ...
Date d2 = ...

long sum = d1.getTime() + d2.getTime();

Date sumDate = new Date(sum);

The code uses the .getTime() method that returns the number of milliseconds since the epoch. Needless to say the Date class has a lot of problems and should be avoided when possible.

Do you want to sum other types instead?

Update: for Calendar, I would do the following (based on javadocs):

Calendar c1 = ...
Calendar c2 = ...
long sum = c1.getTimeInMillis() + c2.getTimeInMillis();
Calendar sumCalendar = (Calendar)c1.clone();
sumCalendar.setTimeInMillis(sum);

UPDATED: As Steve stated, this works if the Date you presented here assumes that the second date is with respect to the Java epoch. If you do want to start with year "0", then you need to account for that (by subtracting your epoch time).

Don't sum the time in millis of the two dates!

Date d1 = new Date();
Date d2 = new Date();
Date dTotal = new Date(d1.getTime() + d2.getTime());
System.out.println(dTotal); // Incorrect! Misses about 1970 years.

Just clone the Calendar and add the datetime parts one by one.

Calendar c1 = Calendar.getInstance();
Calendar c2 = Calendar.getInstance();
Calendar cTotal = (Calendar) c1.clone();
cTotal.add(Calendar.YEAR, c2.get(Calendar.YEAR));
cTotal.add(Calendar.MONTH, c2.get(Calendar.MONTH) + 1); // Months are zero-based!
cTotal.add(Calendar.DATE, c2.get(Calendar.DATE));
cTotal.add(Calendar.HOUR_OF_DAY, c2.get(Calendar.HOUR_OF_DAY));
cTotal.add(Calendar.MINUTE, c2.get(Calendar.MINUTE));
cTotal.add(Calendar.SECOND, c2.get(Calendar.SECOND));
cTotal.add(Calendar.MILLISECOND, c2.get(Calendar.MILLISECOND));
System.out.println(cTotal.getTime()); // Correct!

Needless to say, JodaTime is smarter and cleaner with this.

As always, I would recommend the Java 8 date/time APIs or Joda for date/time work, since they are much more powerful and intuitive.

You can add durations and periods to a DateTime object trivially. You can add minutes/seconds/months equally easily.

However, you can't add two dates directly, since that doesn't really make sense. This is a powerful illustration of why Joda is a help - it stops you doing stuff that you really shouldn't be doing.

You want to do getTimeInMillis() on both those Calendars so you'll have two honest-to-goodness long values you can add up. You can then take the sum and stash it in a new Calendar using that Calendar's setTimeInMillis() method.

Whether you want to add two Calendars as shown above or two Dates as shown in notnoop's answer is up to you, of course. The effect is similar, it just depends on what you want to do with the result. A Date is mostly just good for storing and/or converting to a String for printing out or displaying, whereas a Calendar will let you fiddle with the individual time values should you so choose.

As others have mentioned, you're committing some conceptual no-no's in using a Date or Calendar, which are meant to store "real" dates and times, e.g. ones in the 20th or 21st century, as intervals, i.e. time spans. The classes in the standard Java library don't give you really useful tools to handle this, which is why the Joda classes were developed. All the cool kids in date/time processing use those; but on the other hand that involves downloading and managing a 3rd party library.

notnoop answer is definitely correct. However, if you are going to do lots of processing of dates, times and intervals, I suggest that you look at class DateUtils in apache commons lang and at joda-time library.

JDK7 will come with better support for some of the features that joda-time provides. Just saying ... it might be a consideration if your app makes heavy usage of this stuff.

You need to define your EPOCH. The Java epoch (like Unix) is 1 Jan 1970 GMT/UTC. I assume you think you're adding ten months, 3 days and some odd hours from 1 Jan 0000 but you have a epoch offset until 1970. The maths may not necessarily work.

Use Calendar or Joda (as mentioned). If you just simply want to add a number of seconds and days (&c) then feel free to add said # of milliseconds to your first date object.

I am occasionally guilty of this practice too, storing time interval in a date object and using getTime() as suggested by notnoop.

It works. Contrary to certain opinion, it certainly works. I just ignore that the interval could be representative of an unintended date. It is a quick and dirty way for me to add an interval, say, [6 years, 6 months, 6 days, 6 hours, 6 minutes, 6 seconds] to a date.