Thanks Karel, I'm getting there now! Naturally, I haven't yet figured out how to really control all the possibilities (I can tell they're endless) but for now, I've got my numbers rolling which is basically what I needed.
You might also want to take a look at Dan Ebbert's Expressioneering Design Guide. He's got some very useful counters and clocks driven with expressions. You'll find them in the table of contents. I use them all the time.
One more thing. Be careful with your choice of fonts. Counters like Monospaced fonts. Otherwise the numbers start jumping around all over the place.
You might also want to take a look at This Thread and the odometer type counter that I described. There is a sample project included.