Abstract: An electronic control system for limiting the energy used for building temperature control during unoccupied hours and efficiently bringing the building temperature to a predetermined level at the time occupancy commences is disclosed. A cyclic clock starts a fixed period auxiliary timer a predetermined time period before building occupancy is to commence. The output of the auxiliary timer is electronically modified and compared with an electronically modified temperature signal related to either the environment surrounding the building (for heating systems) or the environment inside the building (for cooling systems). The result of the comparison is used to control the building's heating and/or cooling system in a manner such that the temperature of the building is raised or lowered to a predetermined level when occupancy commences.