Well I think it depends on your Events, you can still keep it all to one event sheet but put the timer event in a Group, and Set the group active whenever you want the timer to start, for example group called "Countdown".
Example:
On Condition Met --- System --> Set Group "Countdown" Active
Else ---> Set Group " Countdown" Inactive
Ofcorse you have to add that under another event like compare variable or so.
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I am sure there are other ways but this what I can think of at the moment