I don’t know how your whole script works but if this is a server script and timer is defined outside of the function, calling startTimer() multiple times will cause the timer to speed up, because your function will run multiple times.
This is a server script but startTimer() is only called once when the frisbee is equipped and I set timer=0 just before calling it. I guess putting timer=0 in startTimer() makes more sense but it should have the same result.