You could set a relay bit whose contact is used to turn ON/Off a timer in a separate ladder circuit.
Alternatively, you could start a timer by assignment its PV to a value you wish for it to start counting down:
E.g. TIMERPV[1] = 100 ' 10.0 second timer.
To clear a timer within a custom function without using a ladder circuit, you can assign the PV to -1 and use CLRIO to clear the timer.
E.g If timer #2 is called "T2", then:
TIMERPV[2] = -1
CLRIO T2
The above two statements resets a timer.