Forget about Calendar and anything else Java-specific or even programming specific for now. You need to write down in English (or whatever your native language is) precisely what steps you would use to go about doing this "manually." Once you've got that--and the steps are very simple and very precise--then you can work on figuring out how to translate it to
Java.
When you get stuck,
TellTheDetails(←click) about your efforts and where you're having problems.