[Zope] Catching a Zope DateTime Error

I have a number of variables of type :date. they get converted to ints on the target script, which works fine, but I need the script to know whether it's a valid DateTime. I have tried catching an exception, but it doesn't throw an exception, it creates a ZOPE error. It doesn't seem to work in a dtml-if statment either. I was wondering how I can handle this error. Regards, Matthew Parslow.