If I have a date in date format, how can I match a value in the date.

For example, in cell A1 I type in 6/30/06. This is the month of June.

In Cell B1, I want to create a formula that says essentially "If the Date is
in the month of June then add 31 days"

So I get 7/31/06 in cell B1.

How do I do this? Thanks very much.