chris8644:
That does not effect it atall, it's the delays within :
No, not directly. However if you use variables to track if the fan should be on and how long it has been on, then you can certainly use millis().
You can setup state variables for things like a fan. Your functions set the state variable to be on or off.
In your main loop() you check those states and set the fan accordingly. You use a variables to track how long the fan has been on. When it has been on for long enough, set the state back to off.
It changes the logic from direct instruction sequence to more of a state machine.