Question:
Is it permitted to adjust a timer (Shabbos clock) so that a light or appliance will turn off at a different time – earlier or later – than originally planned?
Discussion:
Several halachic concerns are involved: the melachah of mav’ir (Kindling), makeh bepatish (finishing a vessel), and whether the timer is muktzeh. The practical halachah is as follows: Many poskim rule stringently and forbid adjusting a time clock under all circumstances. Other poskim are more lenient and permit adjusting the timer so that the lights are turned off or on later than originally planned, reasoning that extending the present state is merely allowing the status quo to continue. However, even these poskim suggest not doing so unless there is a pressing need. All poskim agree one may ask a non-Jew to adjust a timer for a mitzvah or to avoid major loss.