Could you use a web viewer and a Javacript based timer?
We probably need more info - are you just looking to do a countdown or view a FM timer value?
There is a technique to see the ongoing time using web viewer.
You could use this link: https://fmdev.filemaker.com/message/156095#156095
and go down to the reply of Taylor Sharpe
Otherwise, if you need a cowntdown using a non interrupted script, you could use the "pause" script step between each cowntdown value.
Set field [ time = 5:00 ]
Pause [1 sec]
Set field [ time = time - 1]
Exit loop if [time = 0]