If you mean a java.util.Date object, then you should really be using a java.util.Calendar object. This has set and get methods that take constants representing months, hours, etc
For more information on using any of the J2SE classes, The API Specification is a good place to begin to understand what a class or instances of a class do and how to use them. Of course, asking questions and searching past conversations at JavaRanch ain't bad either.