How to get number of days between two calendar instance?

In Java 8 and later, we could simply use the java.time classes.

hoursBetween = ChronoUnit.HOURS.between(calendarObj.toInstant(), calendarObj.toInstant());

daysBetween = ChronoUnit.DAYS.between(calendarObj.toInstant(), calendarObj.toInstant());

Try the following approach:

public static long daysBetween(Calendar startDate, Calendar endDate) {
    long end = endDate.getTimeInMillis();
    long start = startDate.getTimeInMillis();
    return TimeUnit.MILLISECONDS.toDays(Math.abs(end - start));
}