[Zope] What kind of object is bobobase_modification_time?

Its acting really weird. <!--#var "bobobase_modification_time.Date()"--> doesn't work (produces an attributeError) but <!--#var bobobase_modification_time fmt=Date--> does. What I really want to do is <!--#if "(bobobase_modification_time + 60) > ZopeTime()"--> but that causes a type error. Doing ZopeTime(bobobase_modification_time) doesn't work either. I thought bobobase_modification_time was a ZopeTime object. Whats the story?