[Zope] default date unknown

[John Hunter]

I have a date property and need to support an unknown date type.  If I
leave the field blank, I get a syntax error when I try and save the
properties (no more info on error yet, see previous post on error_tb
in manager interface).

    Error Type: AttributeError
    Error Value: SyntaxError instance has no attribute 'replace'

I would be happy to use '' or None.  I found this post
http://mail.zope.org/pipermail/zope-dev/2001-October/013476.html to
handle None.  Is this the preferred approach? 

What is the best/easiest way to support a default unknown date?  I can
also use 0001-01-01 but this is a bit hackish.

Zope 2.7
JDH