Microwaves - Setting Cooking Time Using Selector Knob
Some microwaves have a selector dial to set or adjust cooking time instead of numbered keypads. On most models, the initial microwave time can be set in 15 second increments. Once the microwave starts running, however, any turn of the dial change the time by 5 seconds (clockwise increases, counterclockwise decreases).
For example: To heat something for 20 seconds, turn the dial to 15 seconds. Once you press in on the dial to start the microwave, turn the knob clockwise to add 5 seconds to the cooking time currently displayed.