Sum Two Dates in Java

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).

Combine two type dates into one

Use a Calendar object:

private static Date combine(Date date, Date time) {
    Calendar cal = Calendar.getInstance();
    cal.setTime(time);
    int hour = cal.get(Calendar.HOUR_OF_DAY);
    int min = cal.get(Calendar.MINUTE);
    cal.setTime(date);
    cal.set(Calendar.HOUR_OF_DAY, hour);
    cal.set(Calendar.MINUTE, min);
    return cal.getTime();
}

Test

Date date = new SimpleDateFormat("yyyy-MM-dd").parse("2017-01-28");
Date startHour = new SimpleDateFormat("HH:mm").parse("12:59");
System.out.println(combine(date, startHour));

Output

Sat Jan 28 12:59:00 EST 2017

How can I sum up two ZoneOffset's?

I understand your question this way (please check if correct): You’ve got one ZonedDateTime with a usual offset from UTC. I will call it dateTimeWithBaseOffset. And you’ve got another ZonedDateTime with an offset relative to the offset of the former ZonedDateTime. This is really incorrect; the designers of the class decided that the offset is from UTC, but someone used it differently from the intended. I will call the latter dateTimeWithOffsetFromBase.

Best of course if you can fix the code that produced dateTimeWithOffsetFromBase with the unorthodox offset. I am assuming that for now this will not be a solution you can use. So you need to correct the incorrect offset into an offset from UTC.

It’s not bad:

    ZoneOffset baseOffset = dateTimeWithBaseOffset.getOffset();
    ZoneOffset additionalOffset = dateTimeWithOffsetFromBase.getOffset();
    ZoneOffset correctedOffset = ZoneOffset.ofTotalSeconds(baseOffset.getTotalSeconds()
            + additionalOffset.getTotalSeconds());

    OffsetDateTime correctedDateTime = dateTimeWithOffsetFromBase.toOffsetDateTime()
            .withOffsetSameLocal(correctedOffset);
    System.out.println(correctedDateTime);

Using your sample date-times this prints

2017-12-28T18:30+08:00

If you want the time at UTC:

    correctedDateTime = correctedDateTime.withOffsetSameInstant(ZoneOffset.UTC);
    System.out.println(correctedDateTime);

This prints the datetime you asked for:

2017-12-28T10:30Z

For a date-time with an offset, we don’t need to use ZonedDateTime, OffsetDateTime will do and may communicate better to the reader what we’re up to (ZonedDateTime works too, though).

Java, Calculate the number of days between two dates

Note: this answer was written in 2011. I would recommend using java.time now instead of Joda Time.

Well to start with, you should only deal with them as strings when you have to. Most of the time you should work with them in a data type which actually describes the data you're working with.

I would recommend that you use Joda Time, which is a much better API than Date/Calendar. It sounds like you should use the LocalDate type in this case. You can then use:

int days = Days.daysBetween(date1, date2).getDays();

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