If a person set the light in his dining room on a timer before Shabbat, and during the meal he realizes that he will need the light in the room for 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?
Chacham Ovadia Yosef rules that one may adjust a timer on Shabbat to extend the current status. We do not consider the timer Muktzeh, and thus forbidden to be handled on Shabbat, because of the basic rule that says, "Ein Muktzeh Be'mechubar" – something attached to the ground cannot obtain the status of Muktzeh. Since the timer is attached to the wall of one's house, it is deemed "Mechubar" ("attached" to the ground) and therefore cannot become Muktzeh. Furthermore, one could overcome the issue of Muktzeh by verbally stipulating once each year that he does not deem the timer Muktzeh on Shabbat, and he wishes to use it on Shabbat for the purpose of adjusting the time. This condition is effective in avoiding the prohibition of Muktzeh.
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 would constitute "Geram Kibuy" (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. However, Chacham Ovadia makes an exception in cases involving a sick patient for whom the presence of light causes discomfort. In such a case, one may indirectly turn off the lights by switching the timer to an earlier time than for which it had originally been set.
This rule applies as well to cases where the lights are turned off and one wishes to adjust the timer to have the lights turn on earlier or later than the currently set time. Here, too, it would be permissible to adjust the timer to extend the current status, meaning, to have the lights turn on at a later time, while Halacha would forbid adjusting a timer to have the lights turn on sooner than the originally set time. Where, however, the lights are needed for the purpose of a Mitzva, such as if somebody wishes to use the room for Torah learning, then he may adjust the timer to have the lights go on sooner.
Summary: One may adjust a timer on Shabbat to extend the current status, meaning, to have the light turned on or off later than initially scheduled. One may not, however, adjust a timer on Shabbat to have the light turn on or off earlier than initially planned, except when this is necessary for the fulfillment of a Mitzva or for somebody who is ill.
See Halichot Olam, Helek 4, pages 71-72
