[Zope] date: week in year

Hello Zope has several formatting options for displaying date and time (appendix A Zope DMTL Ref). But sadly enough none of them for displaying the number of the week :-( Is there another Zope way ? Is something possible with Python ? However, my Pythonish is about the same level as is my Swahili ... Greetings Gijs Reulen