The 24 hour timer is designed to turn the unit on and off every 15 or 30 minutes depending on what timer is equipped with your model.
The dial has teeth that adjusts the "ON" and "OFF" time. The teeth on the dial should be pulled outward when you want the setting for "ON" , and you push the teeth inward for "OFF".
(please always refer to the manual for step by step instruction).
The timer has as arrow on the center dial that needs to align with the current time of day when you are setting the unit. Then, you use the teeth to set the time you want the unit to be "ON" by pulling a section of teeth outward or move inward for "OFF" time. This can be set multiple times over a 24 hour priod.
Depending on what model you have, some timers have an additional switch to bypass the timer so you can run the unit with the timer setting or turn the unit completely off.