First off, are you using a system Date (java.util.Date) or a database Date (java.sql.Date)? I am assuming the former, but if you are working with a SQL database, you may be talking about the latter.
Either way, you will want to use one of the Calendar classes, probably GregorianCalendar. The Calendar classes all support a method called .add() which will do the job you want.
However, when adding or subtracting time from a Calendar object, you need to tell it what incement of time to add - an hour, a day, a year, a month, whichever. so what you would have is something along these lines:
Calendar cal = new GregorianCalendar(); // default is current time and default zone
Note that I haven't addressed the question of time zone, which you may neecd to do.