java - how to find the number of days difference between two java.util.Date objects using core java api and not Joda Time

here's a method that calculates the number of days between 2 dates:

...
public static long getDateDiffInDays(Date date1, Date date2) {
  long diffInMillis = date2.getTime() - date1.getTime();
  return TimeUnit.MILLISECONDS.toDays(diffInMillis);
}
...

and here's some test code you can use:

...
public static void main(String[] args) {
  // today
  Calendar today = Calendar.getInstance();

  // tomorrow
  Calendar tomorrow = Calendar.getInstance();
  tomorrow.add(Calendar.DAY_OF_YEAR, 1);

  System.out.println(getDateDiffInDays(today.getTime(), tomorrow.getTime()));
}
...

OUTPUT:
1


In Calculating the Difference Between Two Java Date Instances there's a great, general method for getting differences between dates:

public static long getDateDiff(Date date1, Date date2, TimeUnit timeUnit) {
  long diffInMillies = date2.getTime() - date1.getTime();
  return timeUnit.convert(diffInMillies,TimeUnit.MILLISECONDS);
}

and here's some test code:

public static void main(String[] args) {
  Calendar today = Calendar.getInstance();
  Calendar tomorrow = Calendar.getInstance();
  tomorrow.add(Calendar.DAY_OF_YEAR, 1); 

  // Calculate the difference in days. OUTPUT: 1
  System.out.println(getDateDiff(today.getTime(), tomorrow.getTime(), TimeUnit.DAYS)); 

  // Calculate the difference in hours. OUTPUT: 24
  System.out.println(getDateDiff(today.getTime(), tomorrow.getTime(), TimeUnit.HOURS));

  // Now add 6 hours to "tomorrow"'s point in time
  tomorrow.add(Calendar.HOUR_OF_DAY, 6);
 
  // Calculate the difference in hours. OUTPUT: 30
  System.out.println(getDateDiff(today.getTime(), tomorrow.getTime(), TimeUnit.HOURS));
}


Here's a test class you can play around with: http://pastebin.com/PteRYg2d

Thanks also to:

milliseconds to days

Relevant Java API documentation:
http://docs.oracle.com/javase/6/docs/api/java/util/concurrent/TimeUnit.html
http://docs.oracle.com/javase/6/docs/api/java/util/Calendar.html
http://docs.oracle.com/javase/6/docs/api/java/util/Date.html