No is_date() function?

Hi I wrote a serialization function to turn arbitrary PHP variables into Macintosh plist compatible XML but see that there is no "is_date" tester as there is for bool, object, array etc. Is there a relatively simple (and robust) way to detect if a variable is a date? For example: $person = array('name'=>'bob', 'sex'=>'male', 'date_of_birth'=> $someDateHere); Thanks for any help Ken