Question: I leave my hotplate on a (manual) time-switch on Shabbos. Can I adjust it on Shabbos if I want it to come on or go off earlier or later?
Answer: Rambam (Shabbos 3:1) and the Shulchan Aruch (OC 252:1) write that one can begin a melacha on Friday even though the action will continue running into Shabbos. Thus, R’ Yaakov Breisch (Chelkas Yaakov OC 71), R’ Shlomo Zalman Auerbach (Minchas Shlomo 1:13), R' Eliezer Waldenberg (Tzitz Eliezer 1:20:9), R’ Yehoshua Neuwirth (Shemiras Shabbos Kehilchasa 13:26) and R’ Ovadia Yosef (Yabia Omer OC 3:17) write that one may set timers before Shabbos.
R’ Moshe Feinstein (Igros Moshe OC 4:60), however, disagrees, writing that one may only use timers for one’s lights. Firstly, timers are akin to instructing a non-Jew to perform a melacha on one’s behalf which is prohibited. While the Gemara does allow certain actions to be set up beforehand to run on Shabbos, that only applies when the action process began before Shabbos. Additionally, it undermines the sanctity of Shabbos and had such technology at the time of chazal, they would have prohibited its use. As people always had non-Jews come into their houses to light and extinguish their lights, lights remains an exception (See Rema OC 276:2).
R’ Moshe Feinstein (Igros Moshe OC 4:91:5; YD 3:47:4), R’ Yitzchak Yaakov Weiss (Minchas Yitzchak 2:110; 3:37), R’ Binyamin Zilber (Az Nidberu 3:25; 4:46:7) and R' Eliezer Waldenberg (Tzitz Eliezer 1:20:9) hold that timers are muktza and must therefore not be adjusted on Shabbos.
R’ Shlomo Zalman Auerbach, R’ Yehoshua Neuwirth (Shemiras Shabbos Kehilchasa 13:28) and R’ Ovadia Yosef (Yabia Omer OC 3:18:2) hold that timers aren’t muktza, however. Thus, while one isn’t allowed to adjust the timer to switch on or off earlier than planned, one may adjust the timer to prevent or delay the timer from switching the appliance off by pulling pins out (Minchas Shlomo 1:13). Likewise, one may adjust the timer to prevent or delay an appliance that is off from coming on.
In conclusion, one may adjust a manual timer on Shabbos if necessary, but only to extend the current on or off status.