Diagnosis
The microcontroller that handles the timer function is glitching. This can be caused by a minor power fluctuation corrupting its memory or, less commonly, a failing capacitor on the board that's not holding a stable charge for the clock circuit.
How to Fix It
Perform a full factory reset. Unplug the fan from the wall. Press and hold the Power button on the control panel for 15-20 seconds (with it still unplugged) to drain any residual power. Then, plug it back in. This clears the volatile memory on the main chip. Set the timer again and see if it holds.
If That Didn't Work
If the timer is still erratic, the real-time clock circuit on the control board is faulty. There's no user calibration or fix for this. Your workaround is to simply not use the timer function and control the fan manually or with a smart plug instead.
Heads Up
Using the fan on the same circuit as a large appliance that cycles on and off (like a fridge or AC unit) can introduce small power surges that might confuse the timer circuitry. Try plugging it into a different circuit if the reset doesn't stick.