Preferably, by avoiding the GregorianCalendar entirely. Aside from being needlessly complex, it doesn't actually have methods which will help you - except in the most ineffecient manner possible, which we won't get in to.
All you need is a SimpleDateFormat to parse the date (European style, apparently), plus the Date class and some simple math:
I calculated days rather than months since there is no single precise definition of how long a "month" is.
You should of course adapt this to your needs.
This would probably have fit better in "Intermediate" than either Beginner or Advanced - but I don't really feel like moving it again.
[ February 05, 2002: Message edited by: Jim Yingst ]