How to make a code in language processing for making a chronometer.

By default, when the code is run, the chronometer should count forward every 1/60 of second.

When a mouse key is pressed, the chronometer will start counting reverse.

The chronometer style is: HH:MM:SS:CC; where HH,MM,SS and CC show 2 digits hours, minutes, seconds and 1/60 seconds respectively.

When the chronometer is counting reversely and reaches zero, it should not be negative. It means that when the chronometer is reduced to get negative values, its value should be stopped on zero.