The Shulchan Aruch (Rabbi Yosef Karo,1488-1575) writes (Orah Haim, 252) that one could start a Melacha (an activity forbidden on Shabbat) on Ereb Shabbat even though the Melacha will be completed by itself on Shabbat. He gives an example of one who placed wool in a dye to start the dyeing process before Shabbat that he is allowed to keep it in the dye even when Shabbat starts. Hacham Ovadia Yosef writes that one can set a timer before Shabbat to turn on and off a light, an electric fan or a heating system during Shabbat. Since the person’s involvement occurs only before Shabbat, and not during Shabbat, the timer’s operation on Shabbat does not entail any violation. Many argue on this ruling including Rabbi Masliah Mazuz and Rabbi Moshe Feinstein (1895-1986) that setting a timer before Shabbat to turn on and off a device during Shabbat is forbidden for it looks like a "Zilzul"- degrading of Shabbat.
If a person set the light in the dining room on a timer before Shabbat and during the meal he realizes that he will need the light in the room longer than he had expected, may he adjust the timer so that the lights will turn off later than initially scheduled? Conversely, if a timer is set to turn lights off in a room at a certain time and a person wishes to go to sleep before that time, may he adjust the timer so that the lights will be turned off sooner?
Hacham Ovadia Yosef rules that one may adjust a timer on Shabbat to extend the current status. We do not consider the timer Mukseh, and thus forbidden to be handled on Shabbat, because of the basic rule that says "Ein Mukseh Bemehubar" – something attached to the ground cannot obtain the status of Mukseh. Since the timer is attached to the wall of one’s house, it is deemed Mehubar (attached to the ground) and therefore cannot become Mukseh. Furthermore, one could overcome the issue of Mukseh by verbally stipulating that he does not deem the timer Mukseh, and he wishes to use it on Shabbat for the purpose of adjusting the time. This condition is effective in avoiding the prohibition of mukseh.
Hence, one may extend the timer so that the lights in the room will remain lit for a longer period than for which it had initially been set.
In the converse situation, however, it is forbidden to adjust a timer so that the current status will end sooner, as this will constitute an "indirect extinguishing". Thus, one may not adjust a timer to have the light in his room turn off sooner than the time for which it had originally been set.
Summary: One may set lights and appliances on timers before Shabbat so that they to turn on and off during Shabbat. One may adjust a mechanical timer on Shabbat to extend the current status, meaning to have the lights turned on and off later than initially scheduled. One may not, however, adjust a timer on Shabbat to have the light turn on and off earlier than initially planned.
