date validations

I have two parts of a question. First I have several dates in my app
which aren't required, so I don't think I can use a date_select right?
There doesn't seem to be a way to give the user an option to *not*
select a date.

So, assuming I need to just give the users a form field, anyone have a
good piece of code for checking the validity of a date, allowing
various forms, i.e.:
12.12.2007
12/12/2007
12-12-2007

Any suggestions?

Thanks, Joe.