Have you read what I posted in post #7 Countdown timer with potentiometer - #7 by StefanL38
On an alarm-clock you have a state-change from
- alarm-time not (yet) reached
to - alarm-time reached
what is your state-change that makes your code know:
the count-down time starts here
= switch on LED and from now on
and in 30 min. 1h, 2h in the future switch LED off
You could use the potentiometers value that changes from beeing outside the range
(potVal > 300 && potVal <= 500)
to be inside this range. But this would mean:
Your code is running very very fast as soon as you enter the range 300-500 this would be taken as the trigger.
You would not be able to choose a value
So there must be something additional that is used as the trigger to say
the count-down time starts here
My suggestion: a button press.
best regards Stefan