An electronic multiple-setback thermostat for residential and commercial use includes an internal clock having a week-long cycle. The thermostat is programmed by the operator, usually manually actuable input devices, to control the furnace/air conditioning system to achieve a schedule of temperatures over the weekly period by simply inputting the temperature desired to be attained at the time of entry. Each temperature entry may be programmed to reoccur on a daily basis or to modify the program on a once-a-week basis. The thermostat employs an internal clock which controls memory addresses so that each memory location is representative of a short time period during the weekly cycle. A desired temperature signal is entered in a memory location determined by the clock state at the time of entry and is read out to the control system each time that time state reoccurs. The external temperature sensor employs a thermistor controlled oscillator that is used as the system clock source during failures of the primary power source.