[Zope] Using DTML or Python in ZClass Properties?

I have a ZClass with a property that is a date. I would like that property's value to contain the current date as a default, subject to user override. In the ZClass Property Sheet editor, I can't seem to get a DTML expression to work. Maybe I'm formatting it wrong, but I keep getting an invalid date string kind of answer. One document I read suggested that I could put the date into the object's constructor method by calling manage_editProperties, but it made a reference to "if you add fields for properties to the HTML form on the management screen" and that was just imprecise enough that it eludes me. Any ideas?