[Zope-dev] Can I create dtml to calculate a date 30 days ago?

I would like to do the following: <dtml-call "REQUEST.set('magicdate', DateTime()-days)"> where days is a variable set in a form in a higher level web page. It does not work, complains about "bad operand for unary -". Is there a way in DTML to do this? (And if so, what is it?). I am usually pretty good at trying quoting variants until I stumble accross one that works, but I just can't find it this time. Thanks Jim