[Zope] Plone Calendar not working with Zope 2.7.1

Hi! I have just upgraded to Zope 2.7.1 and now the calendar isn't working anymore. It always shows January instead of the current month. I couldn't find out which change in DateTime causes this problem. Zope 2.7.1 Plone 2.0.3 Regards, Florian Schulze