[Zope] Calendar tag

[Cornelis J. de Brabander]

I want to implement a  editable calendar. As a first step I installed the
Calendar Tag product and followed the Jon Udell How-To almost to the letter
(except for user Flintstone). I created a external method called
editCalendarProperty referring to a function with the same name and a Python
module Calendar.CalendarItem (CalendarItem.py was placed in a Extensions
subdirectory of the Calendar product directory). Without 'date' properties
the document renders fine. After editing the calendar a property is added to
the document with the correct value, but the document fails to render
anymore due to:
Error Type: TypeError
Error Value: not enough arguments for format

Traceback (innermost last):
  File D:\Zope212\lib\python\ZPublisher\Publish.py, line 214, in
publish_module
  File D:\Zope212\lib\python\ZPublisher\Publish.py, line 179, in publish
  File D:\Zope212\lib\python\Zope\__init__.py, line 202, in
zpublisher_exception_hook
    (Object: ElementWithAttributes)
  File D:\Zope212\lib\python\ZPublisher\Publish.py, line 165, in publish
  File D:\Zope212\lib\python\ZPublisher\mapply.py, line 160, in mapply
    (Object: index_html)
  File D:\Zope212\lib\python\ZPublisher\Publish.py, line 102, in call_object
    (Object: index_html)
  File D:\Zope212\lib\python\OFS\DTMLDocument.py, line 166, in __call__
    (Object: index_html)
  File D:\Zope212\lib\python\DocumentTemplate\DT_String.py, line 502, in
__call__
    (Object: index_html)
  File D:\Zope212\lib\python\Products\Calendar\CalendarTag.py, line 282, in
render
TypeError: (see above)

By inspecting CalendarTag.py I have tried to figure out what might be the
problem, but my Python competence is simply still not sufficient. Does
anybody have a clue?
tia, cb.