Pass parameters based on Javascript validation

I have a client-side javascript that will not allow a user to proceed unless the date specified in the date field is the current date....pretty simple, right? (I am using 3 dropdowns(m/d/y))

What I want to do is pass a parameter if the user tries to choose a date other than the current date. ie: when the user is alerted, set a variable, which will be passed in the form post method when the user is allowed to proceed.

I imagine that this a simple routine - a hidden value in a form - but how do I get that flag variable to be passed?

--
Mh