Subtract 6 hours from an existing Date object java (midnight corner case)

Question

I\'ll be using the Calendar api for this. My primary concern is that

            Date birthDate = (...say Apr 20th 0300hrs)
            Calendar ca         


        

Answer 1

Second parameter in Calendar's .add method is indeed the delta, or the change, so yes, this would subtract 6 hours.

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Answer 2

Yes.

Reference for calendar.add

http://www.tutorialspoint.com/java/util/calendar_add.htm

Answer 3

No.

After running the following code:

Date birthDate = (...say Apr 20th 0300hrs)
Calendar cal = Calendar.getInstance();
cal.setTime(birthDate);
cal.add(Calendar.HOUR, -6);

you are guaranteed that cal is set to six hours before 'Apr 20th 0300hrs', but not that it is set to 'Apr 19th 2100hrs'.