Calculate number of days between two dates

Computing number of days between two dates is simple mathematics. This is how it is done.

public int differenceInDays(Calendar start, Calendar end) {  
    long ms = Math.abs(start.getTimeInMillis() - end.getTimeInMillis());
 
    return (int) (ms / (1000 * 3600 * 24));
}

But does this work in all cases? Not exactly. Take a look at this code:

public Calendar newDateInstance(int year, int month, int day) {
    Calendar cal = Calendar.getInstance();
    cal.set(Calendar.YEAR, year);
    cal.set(Calendar.MONTH, month - 1);
    cal.set(Calendar.DAY_OF_MONTH, day);
    cal.set(Calendar.HOUR_OF_DAY, 0);
    cal.set(Calendar.MINUTE, 0);
    cal.set(Calendar.SECOND, 0);
    cal.set(Calendar.MILLISECOND, 0);

    return cal;
}

Calendar start = newDateInstance(2011, 3, 13);
Calendar end = newDateInstance(2011, 3, 14);
System.out.println(differenceInDays(start, end));

This code prints 0. Why? Because on 14th March morning Daylight Saving starts and an hour is lost. One solution I use is to add 1 hour to end date to avoid loosing a day. Here's how above code is rewritten.

private void resetTime(Calendar cal) {
    cal.set(Calendar.HOUR_OF_DAY, 0);
    cal.set(Calendar.MINUTE, 0);
    cal.set(Calendar.SECOND, 0);
    cal.set(Calendar.MILLISECOND, 0);
}

public int differenceInDays(Calendar start, Calendar end) {  
    resetTime(start);
    resetTime(end);
 
    // This is to avoid loosing a day when daylight saving starts(14th March)
    end.set(Calendar.HOUR_OF_DAY, 1);
 
    long ms = Math.abs(start.getTimeInMillis() - end.getTimeInMillis());

    return (int) (ms / (1000 * 3600 * 24));
}