Depending on the model of the portable air conditioner you own the way to set the timer may vary. The timer can be set where you choose a delay on time or you choose how many hours the unit will run and when to turn off. To set these times press the timer button on your display and use the "-" and "+" buttons to set the desired numbers of hours (please note: some units are 12 hour timers and some are 24, refer to the manual for your specific model). If you are choosing a delay start time the unit must be off while you are setting the time. If you are choosing when the unit should turn off the unit must be operating while setting the time.  NOTE: these time settings are one time settings.