Here's a bare bones example of using Numbers generators to build a clock... this one is a countdown clock which uses oscillators that can drive the Numbers Values in negative directions.
The generator is 1 minute and 1 second long (the last second is a "still" of 00:00:00 -- you can add more time if you like.) This generator utilizes a Build In Mandatory marker to fix the timing (no stretching) but it can't be cut to shorten the countdown to less than 1 minute (you'll need to cover it or place it in a compound clip to do that). The Minutes value is the only parameter available. You design longer periods by adding multiple instances of the generator to the timeline. You will need to cut the last second off each but the last one (but it's an easy task.)
About the Oscillator behaviors:
Wave shape is sawtooth (ramps from max to min and restarts at max again.) I have the phase set to -π (but any multiple of π will work just as well.) The amplitudes were set to -30 (directional) and the original Value of the Number generator was set to 30 (creating an offset to make the amplitude go from 59 to 0 [it's at 60 for 1 frame]). The Speed was set for 60 for the "frames" value and 1 for seconds.
http://sight-creations.com/fxexchange/TimeOscillator.zip
HTH