How can I generate a single timer event in a loop?

I have a simple PID script running inside of a timed loop to control a heating process.  It works perfectly except that I cannot find a clean way to start the timer when it reaches the setpoint.

Right now, the loop just runs for a set amount of time (e.g., thirty minutes).  I want it to run for a set amount of time after the heating reaches the setpoint.  I can think of several ways to fudge this, for example with a couple of shift registers and a case structure or with a second loop, but these options seem very clunky.

I am sure there is a better way.  Can I somehow create an event within the loop that only happens once, for example, to give a timer value one time when a boolean value changes and never rewrite it?

Thanks in advance for your help.