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Asked: 2026-05-11T17:42:53+00:00

Please your opinion on the following code. I need to calculate the diff in

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Please your opinion on the following code.

I need to calculate the diff in days between 2 Date objects. It is assured that both Date objects are within the same TimeZone.

public class DateUtils {
public final static long DAY_TIME_IN_MILLIS = 24 * 60 * 60 * 1000;

/**
 * Compare between 2 dates in day resolution.
 * 
 * @return positive integer if date1 > date2, negative if date1 < date2. 0 if they are equal.
 */
public static int datesDiffInDays(final Date date1, final Date date2){
    long date1DaysMS = date1.getTime() - (date1.getTime() % DAY_TIME_IN_MILLIS);
    long date2DaysMS = date2.getTime() - (date2.getTime() % DAY_TIME_IN_MILLIS);

    long timeInMillisDiff = (date1DaysMS - date2DaysMS);
    int ret = (int) (timeInMillisDiff / DAY_TIME_IN_MILLIS); 
    return ret;
}

Can you point to a problem that I might have missed ?

EDIT: @mmyers asked if pass my unit test. Well – Yes. But I have no real experience with dates and I know that is a big subject. Posted below the unit test that I’m using.

public class TestMLDateUtils {

@Test
public final void testDatesDiffInDays() {
    TimeZone.setDefault(TimeZone.getTimeZone("UTC"));

    // 00:00:00.000 1.1.1970
    Calendar cal1970 = Calendar.getInstance();
    cal1970.setTimeInMillis(0);

    Calendar tested = Calendar.getInstance();
    tested.setTimeInMillis(0);

    // Add 1 millisecond, date = 00:00:00.001 1.1.1970
    tested.add(Calendar.MILLISECOND, 1);

    assertTrue(DateUtils.datesDiffInDays(cal1970.getTime(), tested.getTime()) == 0);

    // Add 1 second, date = 00:00:01.001 1.1.1970
    tested.add(Calendar.SECOND, 1);
    assertTrue(DateUtils.datesDiffInDays(cal1970.getTime(), tested.getTime()) == 0);

    // Add 1 minute, date = 00:01:01.001 1.1.1970
    tested.add(Calendar.MINUTE, 1);
    assertTrue(DateUtils.datesDiffInDays(cal1970.getTime(), tested.getTime()) == 0);

    // Add 1 hour, date = 01:01:01.001 1.1.1970
    tested.add(Calendar.HOUR_OF_DAY, 1);
    assertTrue(DateUtils.datesDiffInDays(cal1970.getTime(), tested.getTime()) == 0);

    // date = 23:59:59.999 1.1.1970
    tested.setTimeInMillis(0);
    tested.add(Calendar.MILLISECOND, 999);
    tested.add(Calendar.SECOND, 59);
    tested.add(Calendar.MINUTE, 59);
    tested.add(Calendar.HOUR_OF_DAY, 23);
    //System.out.println("D: " + tested.getTime());
    assertTrue(DateUtils.datesDiffInDays(cal1970.getTime(), tested.getTime()) == 0);

    // date = 00:00:00.000 2.1.1970
    tested.setTimeInMillis(0);
    tested.add(Calendar.DAY_OF_MONTH, 1);
    assertTrue(DateUtils.datesDiffInDays(cal1970.getTime(), tested.getTime()) == -1);
    assertTrue(DateUtils.datesDiffInDays(tested.getTime(), cal1970.getTime()) == 1);

    // date = 00:00:00.001 2.1.1970
    tested.add(Calendar.MILLISECOND, 1);
    assertTrue(DateUtils.datesDiffInDays(cal1970.getTime(), tested.getTime()) == -1);
    assertTrue(DateUtils.datesDiffInDays(tested.getTime(), cal1970.getTime()) == 1);

    // date = 00:00:01.001 2.1.1970
    tested.add(Calendar.SECOND, 1);
    assertTrue(DateUtils.datesDiffInDays(cal1970.getTime(), tested.getTime()) == -1);
    assertTrue(DateUtils.datesDiffInDays(tested.getTime(), cal1970.getTime()) == 1);

    // date = 00:01:01.001 2.1.1970
    tested.add(Calendar.MINUTE, 1);
    assertTrue(DateUtils.datesDiffInDays(cal1970.getTime(), tested.getTime()) == -1);
    assertTrue(DateUtils.datesDiffInDays(tested.getTime(), cal1970.getTime()) == 1);

    // date = 01:01:01.001 2.1.1970
    tested.add(Calendar.HOUR_OF_DAY, 1);
    assertTrue(DateUtils.datesDiffInDays(cal1970.getTime(), tested.getTime()) == -1);
    assertTrue(DateUtils.datesDiffInDays(tested.getTime(), cal1970.getTime()) == 1);

    // date = 13:01:01.001 2.1.1970
    tested.add(Calendar.HOUR_OF_DAY, 12);
    assertTrue(DateUtils.datesDiffInDays(cal1970.getTime(), tested.getTime()) == -1);
    assertTrue(DateUtils.datesDiffInDays(tested.getTime(), cal1970.getTime()) == 1);
}
}
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1 Answer

  1. Editorial Team

    • Immediate problem: days can have less than or more than 24 hours due to daylight saving time changes.

    • Secondary problem: normally when people think in days, they really mean “human days” rather than “periods of 24 hours”. In other words, many people would say that 7pm-7am the next day is a difference of a day, whereas 7am-7pm the same day is a difference of zero days. Both are 12 hours. At that point, you really need to know the calendar that is being considered.

    Of course, this may not matter for your situation, but we don’t really know what that is.

    • Third problem: you’re using the built-in calendar API instead of Joda Time. That’s almost never a good idea – it’s horrible and riddled with gotchas and problems. And yes, the regulars here will tell you that’s always part of my answer when it comes to Java dates and times – and for good reason. It’s really that important.

    EDIT: Your test sets the default time zone to be UTC. That’s not really a good idea (especially without resetting it in a finally statement). Time zones are tricky, but you should really think about what values you’ve got, what they mean, and what time zones are involved.

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