adding dates

I hope this makes sense:

If a date is entered into a cell in one column (A) I need 90 days added to that date in another column (B). If there is no date entered into (A) then (B) needs to be blank. I am also looking for a good source (cheat sheet) that I can ref. with excel formulas and symbols (like what does $ do when in a formula).

Thanks for the help.......I waisted a couple hours trying to figure that one out. The links are great as well. This is the best forum site I have visited to date.