[Zope] Date fields in new ZClasses

I'm using Zope 2.3.2 and created a ZClass. I wanted to add a (date) field as a standard property for the class. However, unlike the string properties, I had to include an actual date in the data field. It gave me an error when I tried to leave it blank. Does anyone know why you MUST include a non-null date in a date field of a ZClass? Cheers, Ron