CODEWITHSUNDEEP
javadatedatetime
Pectus Excavatum
Pectus Excavatum

Reputation: 3773

Java, How to subtract Date objects whilst considering DST

I have a piece of code that is used to calculate the number of days between two Date objects, and in most instances, it works fine. However, if the date range between the two objects includes the end of March, the result is always 1 less than it should be. e.g March 31 2014 - March 29 2014 = 1, whereas it should be 2.

I understand that this is due to the fact that March has 30 days of 24 hours and 1 day of 23 hours due to DST, which is the cause of the value being 1 less.

However, I am not sure the best way to account for the missing hour.

   // This was what I have initially
    int numDays = (int) ((dateTo.getTime() - dateFrom.getTime()) / (1000 * 60 * 60 * 24));

   // I have tried rounding, since it should have 23 hours left over, but it didn't actually work.
    int numDays = (Math.round(dateTo.getTime() - dateFrom.getTime()) / (1000 * 60 * 60 * 24));

Any help/pointers would be greatly appreciated.

I am and have to use Java 7 and I am not able to use Jodatime unfortunately.

Upvotes: 0

Views: 106

Answers (1)

dcsohl
dcsohl

Reputation: 7396

Your second example is very close. Your parentheses for Math.round() only surround the subtraction, though, so since that's already an integer (well, a long really), nothing happens, and then you divide. The other problem with your second bit of code is that you are doing integer division which always truncates the part after the decimal point. Try this:

long numDays2 = Math.round((dateTo.getTime() - dateFrom.getTime()) / (1000.0 * 60 * 60 * 24));

(As indicated, I changed the Math.round() parens, and made it floating point division by making the divisor a double.)

As indicated by the comments, though, this is a hack. In particular, it will tell you that there are two days between 6AM March 5 and 8PM March 6. It's probably not really what you want. Try this on for size instead:

    SimpleDateFormat fmt = new SimpleDateFormat("yyyy-MM-dd");
    Calendar cal = Calendar.getInstance();
    cal.setTime(fmt.parse("2014-03-29"));
    long start = cal.getTimeInMillis();
    start += cal.getTimeZone().getOffset(start);
    cal.setTime(fmt.parse("2014-03-31"));
    long end = cal.getTimeInMillis();
    end += cal.getTimeZone().getOffset(end);

    System.out.println((end - start)/86400000.0);

Is it ugly? Yes. Is it weird? Yes. Does it work? Yes (I think so). Note that I'm providing a double as a result; you can apply any rounding you want to this result.

Upvotes: 1

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