You shouldd be getting an error!
Change the first instance of:
Code:
self.TimeSinceLastUpdate = 0
to:
Code:
f.TimeSinceLastUpdate = 0
f is the frame you just created. The second place you reset self.TimeSinceLastUpdate is the frame referring to itself inside the script.
XML in WoW is used for frame creation and not much else (but that can also be done in lua alone).