Internally the timer present-value uses -1 to indicate to the CPU that it is inactive. If the timer-present value is zero it could also mean time-out which may not be your intention?
If you don't reset the timer, but simply assign it a new PV and clear its contact whenever you want to restart the timer then it will count down to zero from the new value and will never become -1 (except after power on reset).
E.g. CLRIO Timer1 ' assuming timer is named Timer1
TIMERPV[1] = 50 ' start a 5 second count down.
