Time switches are available in both mechanical and completely electronic versions. They are commonly utilized in the following applications: aquariums, home lighting (both interior and exterior), ovens, street lights and washing machines. In cold climates, block heaters, battery blankets, and interior car warmers may be plugged into a timer that switches them on an hour or two before planned use of the vehicle, saving energy; outdoor weather-resistant timers are sold for this purpose. Permanently-installed (hard-wired) time switches may be installed to control exterior lighting of buildings. Astronomical time switches are constructed to compensate for the varying length of day, so that lighting turns on at dusk and off at dawn.