DIAGNOSIS: The timer function is software-based, running on the main microcontroller. Glitchy timer behavior points to a firmware hiccup or a faulty real-time clock circuit on the board.
FIX: Perform a hard reset to clear the fan's temporary memory and reload the firmware. Unplug the fan from the wall outlet. Wait a full two minutes (not just a few seconds). While it's unplugged, press and hold the power button on the unit itself for about 30 seconds to discharge any residual power. Plug it back in. Set a timer for a short duration, like 15 minutes, and see if it now behaves correctly. Test both the 1-hour and longer settings.
IF THAT DIDN'T WORK: If the timer is still erratic, there's a known but less common issue where the internal capacitor that maintains the timer memory during normal operation is failing. This isn't something you can fix.
HEADS UP: The timer function is one of the more fragile parts of the firmware on these. If you rely on it, the hard reset is the first and often only tool you have. If that fails, you'll probably have to just use the fan without the timer or contact support.