How to capture invalid date entries

Hi Forum,
I need a way to error check when a user enters an invalid date manually (rather then using date picker).
Let me explain further

In the month of February there are only 28 days. Therefore if i enter into my field 'start_date' 30/02/2010 then the database will simply contain zeros and subsequent view pages that contains the record and field will display an inaccurate date back to the user.
I need a way to check the number of days in a given month for the number of months in the year 2010, 2011 & 2012.
Thanks in advance
JK