[Zope] dtml-calendar in 2.5.1 problem

Hi all, I haven't asked a question for over a year: Zope has been running smoothly for me. I've recently landed a new server, though, and took the opportunity to upgrade from 2.3.1 to Zope 2.5.1. The dtml-calendar tag I had been using breaks in the process. Can someone point me to a fix, or at least point out the problem? Thanks! Anthony