Simple stopwatch

Ive been surfing around at stopwatches, but there all too high tech and hard for me to use, i want just this:

execute function for start

execute function for stop

counting in decimal (no binary or hex or something) with 2 decimals for seconds, and 2 for milliseconds. in the form 33.71 (33 seconds, 71 milliseconds…)

th value 33.71 should then be in a variable (which it probably allready is…)

hope theres something not too insane for this…