You can set the clock to a particular value, either instantly or animated to position (to stay true to the the behavior of a mechanical barrel clock, all values should be animted except resetting to "00:00:00" which should happen instantly):

You can have a stopwatch count up or down:

0 0 : 0 0 : 0 0

The stopwatch also has a zero-stop parameter, so you can use that with a combination of the previous functions to make an effective countdown to zero:

The mechanics behind this is a digit animation function - each digit automatically increments it's neighbour if it gets above its numeric base, or decrements if it gets below 0, so the stopwatch is simply a one-second loop to change the final digit by one.