[Zope] Subtracting DateTimes?????

We backup our database every night at 3:00 AM CT. 15 minutes prior to this (i.e. 2:45 AM) I would like to display a warning. How do I compare the current time <dtml-var "(ZopeTime()).toZone('US/Central')"> to 3:00 AM Central Time ????? <dtml-if "(ZopeTime()).toZone('US/Central') - 0.3"> doesn't work. Any thoughts??? Dan