New owner, old hot tub!

We are in working order, but the high end limit doesn't seem to shut off the heater when it should, so instead of setting it to auto, my hubby and I want to set the timer. By the looks of it, it should be simple. Hmmm...

The dial is extremely hard to turn if we are even suppose to turn it.

Anyone know how to set it so it will come on at 6:30am for an hour or two, and again at 3:30pm for 3 hours? I will assume that each little "tick" is increase the time ON by approximately 15 minutes, so as Winter progresses, we will be able to maintain decent heat.

Please let me know what you know!

Thank you in advance!

Pamela