[Zope] Newbie question

I am currently browsing the API Reference section of the Zope book and am trying simple examples. I am puzzled as to how various methods may be accessed from dtml. For instance the line: <dtml-var expr="getSize()"> works fine, but: <dtml-var expr="dow()"> (which I would have expected to return the day of week) gives an error Obviously I am confused. Could somebody try and put me on the right tracks! TIA Dave.