Simple manual countdown timer, but bugged (fla included)

Hi ppl,
Myself is not much of a programmer, but recently just need to create a countdown timer for work that allows manually add-in the time duration that’s needed right on the interface.
My idea is simple, a reverse frame playback function with a timer. The code did its work except that when the time reaches 0, it didn’t seem to stop the “reverse action”, and therefore I can’t have another go of the timer as that “reverse action” kept kicking in automatically. I’ve tried to add in a stopper, but all as in vain.
Please take a look of my code if any of you happen to have some time for me.
Many thanks!