Timelapse Countdown Timer in After Effects I'm sure someone out there must have done this before, but how do I create a countdown timer with hours, minutes and seconds that starts from a specific time and counts down at 71.928x speed?

71.928x because I took one frame every 3 seconds and I'm playing it back at 23.976fps.

I've tried using text expressions but to no avail.
You will have to do it with expressions, unfortunantly it is the only outside of possibly finding a 3rd Party plugin...

This is a good start, http://www.motionscript.com/design-guide/up-down-clock.html