Abstract: A variable duration clock timer unit which may be adjustably set to energize an electrical circuit for a selected duration for each period of clock operation includes a momentary contact, normally open, switch frictionally mounted at a selected location in a concentric annular groove in a circular clock face below a rotatable clock arm driven by an electric clock motor. The clock arm is adjustable in length and has a free end in the shape of a triangle overlying the annular groove. The switch actuator may be depressed by engagement with the free end of the clock arm to close the switch for the period of time that the free end is in engagement with the switch actuator. Adjustment of the length of the clock arm changes the width of the free end portion of the clock arm engageable with the switch actuator and thus determines the duration of switch closing. The timer unit may be used to automatically control the operation of an electric defrosting circuit of a motor vehicle.