Diagnosis
The timer knob on these can develop a weak spot or feel "detached" internally. It's a mechanical timer, and turning it past 30 minutes usually just means it's set to run for a longer cycle (it can go up to 60 minutes), not that it's broken.
How to Fix It
First, understand how it works. Turning it clockwise sets the time. When it's running, you'll hear it ticking down. 1) To test it, plug in the unit with the basket clicked in. 2) Set the temperature knob. 3) Turn the timer knob to 5 minutes. It should start, and you'll hear the fan and the ticking. If it does that and turns off after 5 minutes, the mechanism is working. The "weird" feeling is likely just wear on the plastic gears inside the knob assembly.
If That Didn't Work
If it doesn't start at all when you set the timer, or doesn't turn off, the internal timer mechanism might have failed. At that point, the repair isn't practical. You'd need to contact Dash support about the issue.
Heads Up
You can manually turn it off at any time by just turning the timer knob back to 0. That's a good way to check food without letting the timer keep ticking down.