Look into GregorianCalendar class. There is an add() method that adds a number of days or months or years or whatever. Use a negative number to subtract.
Now,
Java is definitely messed up when it comes to dates. You've got Date classes in util and sql, and GregorianCalendar. Converting between them can be maddening. Be patient, look for a good book on the subject (Thinking in Java is free here:
http://64.78.49.204/) or hope somebody smarter than me chimes in