how to test string to see if it is a date or time?

I have a form that is passing a date variable, and a time variable. I will be using these for a query, and will use the date function to do some formatting on it. now, I would like to idiot proof it a bit, but can't find any functions for this (hoping something similar to is_numeric exists). does anybody have any commands, or some code that would allow me to make sure a variable is a valid date or time. thanks, Jeff