[Zope] date-Attributes with no value

Hi, I just wondered if it is possible (without an Python-based Product) to have date-Properties that have no value, but None or some equivalent. I have some Date's in my ZClasses that need not to be set if the class is in a certain state, so I would like to omit the input field in the creation form. But I do have to include a hidden input field at least, to avoid Zope complaining about an incorrect Date-String. Also a hidden field using None as content doesn't work, but this gives me an IndexError. It is not a real problem, as I can define any date earlier than 1970-1-1 to be illegal, but I wondered if it is possible to have some value that renders to false in Python. Andreas -- Cheer Up! Things are getting worse at a slower rate.