Difference in days between two dates in Java?

Question

I need to find the number of days between two dates: one is from a report and one is the current date. My snippet:

  int age=calculateDiffer         


        

Answer 1

Illustration of the problem: (My code is computing delta in weeks, but same issue applies with delta in days)

Here is a very reasonable-looking implementation:

public static final long MILLIS_PER_WEEK = 7L * 24L * 60L * 60L * 1000L;

static public int getDeltaInWeeks(Date latterDate, Date earlierDate) {
    long deltaInMillis = latterDate.getTime() - earlierDate.getTime();
    int deltaInWeeks = (int)(deltaInMillis / MILLIS_PER_WEEK);
    return deltaInWeeks; 
}

But this test will fail:

public void testGetDeltaInWeeks() {
    delta = AggregatedData.getDeltaInWeeks(dateMar09, dateFeb23);
    assertEquals("weeks between Feb23 and Mar09", 2, delta);
}

The reason is:

Mon Mar 09 00:00:00 EDT 2009 = 1,236,571,200,000
Mon Feb 23 00:00:00 EST 2009 = 1,235,365,200,000
MillisPerWeek = 604,800,000
Thus,
(Mar09 - Feb23) / MillisPerWeek =
1,206,000,000 / 604,800,000 = 1.994...

but anyone looking at a calendar would agree that the answer is 2.