Turning an electrical appliance on or off is one of the activities prohibited on Shabbat. However, a Jew may program a Shabbat clock to turn on a light on Friday evening at 7:00 PM for Shabbat dinner, and then to turn off the light at 10:00 PM after dinner has ended. The light turns on and off automatically, without human intervention, so its use on Shabbat is permitted. The Shabbat clock must be programmed before Shabbat begins.
A Shabbat clock is connected to the distribution board in a house of Shabbat-observant Jews, and its primary use is to turn on and off the lights in the house. Other uses are disputed among halakhic authorities.
The most common Shabbat clock is a mechanical device. It is connected to an electrical distribution board with a clock. Around the clock is a circle that may be twisted, and little hooks may be attached at any point to designate the moment at which the switch will turn on or off. One type of hook turns the device on, and another type turns it off. Control of the clock may be separated from control of the power sockets.
Digital Shabbat clocks also exist to control the switching more precisely.
Since many poskim permit using Shabbat clocks for electrical appliances, such as air conditioners or ovens for cooking and baking, most companies that sell Shabbat clocks include a feature that allows the clock to operate any device.
The concept of Shabbat clocks forms the basis for halakhic solutions to manage public institutions on Shabbat, such as hospitals and hotels. On kibbutzim and moshavim of Shabbat-obsevance Jews, residents may use Shabbat clocks to operate dishwashers and milk cows.