Shabbat Timer: adding/subtracting time Rabbanit Debbie Zimmerman

In short, yes, poskim such as Rav Shlomo Zalman Aurebach and Rav Ovadia Yosef allow for the situations that you asked about. In general, they agree that on both Shabbat and Yom Tov one may adjust the timer on a shabbat clock to extend the current situation. Therefore:

If the timer is set to go on, one may add time for it to stay on (and delay it turning off).
If the timer is currently off, one may add time for it to stay off (and delay it turning on).

Halachic Reasoning:

It should be noted that not all halachic authorities agree with this position. Rav Moshe Feinstein rules that it is forbidden to adjust a timer on shabbat, adjusting the time the light goes on is a Torah prohibition, while adjusting the timer so the light continues to stay on until the end of shabbat violates the rabbinic prohibition of muktza. (Igrot Moshe YD III, 47)[1]

Rav Moshe Feinstein does not explain the reasoning behind his ruling; other authorities offer their own reasons to prohibit adjusting timers. The gemara discusses a prohibition of adding or removing oil from the reservoir of an oil candle. (TB Beitza 22a, Shabbat 29b) Rosh rules that doing so violates a biblical prohibition – that of lighting or extinguishing a fire. (Beitza 2, 17) Rav Ovadia Hedaya compares this to changing a timer on shabbat, as one causes a change in the electrical current that will cause it to go on or off, therefore ruling that doing so violates a biblical prohibition. (Yaskil Avdi 7, 23)

Rav Melamed brings several reasons why Rosh’s ruling should not be applied to Shabbat timers. For example, Mishna Berura (514:20, 25) explains that Rosh only prohibits tampering with the fuel source itself to extinguish the fire but does not prohibit using an external object, such as partially burying a candle in sand on Yom Tov so that it is extinguished when it gets to a certain point. (Harchavot Peninei Halacha Shabbat 17:6) Rav Melamed explains that since Shabbat timers are external to the appliance the Rosh’s opinion should not be used to prohibit adjusting one on Shabbat.

Indeed, there are several important opinions that prohibit adjusting timers on Shabbat; yet there are also reliable poskim that permit some adjustments and offer compelling arguments against those that prohibit them, such as the example from Rav Melamed above.

Rav Ovadia Yosef rules that since the shabbat timer is external from the electric appliance, extending the current state of the timer merely removes an impediment and cannot be compared to tampering with the oil in a light’s reservoir. (Yabia Omer III, 18)

Similarly, Rav Shlomo Zalman Aurebach allows for certain timer adjustments as they are external to the electrical device. He compared extending a shabbat timer to closing a window so the wind does not extinguish a candle. While closing the window extends the time the candle is lit it does not change the candle or source of ignition itself, it just prevents an external force from exerting itself and extinguishing the candle. As there is no change to the light or even the electrical current, rather one merely prevents the shabbat timer from changing the state, they claim this is a more fitting example. (Minchat Shlomo I, 13( Rav Aurebach adds that since this is allowed, the “clicks” on the shabbat timer are not considered muktza.[2] (Minchat Shlomo II, 27)

Changing the current state

Although you did not ask, it seems pertinent to note why one may extend the current state but making the appliance go off or on earlier is not as easily permitted. Such an act affects the electricity, even though it is delayed. Many consider such a delayed change to fall under the halachic category of gramma, indirect causation.

The gemara rules that even though extinguishing a fire that does not endanger lives is prohibited, one may cause such a fire to be extinguished by putting ceramic jugs of water in its path. When the fire gets close the heat should cause the jugs to burst, which will then extinguish the fire. (TB Shabbat 120b) This is a classic example of gramma, which is rabbinically prohibited under normal circumstances but poskim discuss special cases when it is allowed, such as the fire that will cause significant financial loss. (Rema OC 334:22)

We will not go into the precise mechanism of gramma here, suffice it to say that as extending the timer does not directly affect the light or appliance and does not immediately cause any state to change, many rule that in certain cases changing the timer so it will go on earlier is considered gramma and therefore allowed in certain cases.

In notes to Shmirat Shabbat K’Hilchata the author expands that in certain specific cases – such as for a sick person or for a mitzva – one may hasten the changing of the state and cause the light or heater to go on earlier – as long as the change is not immediate. (2nd edition, Chapter 13, notes 90-93)

While Shulchan Aruch allows one to use gramma to extinguish a fire, Rema limits this to cases when there is a significant financial loss. (Rema OC 334:22) Therefore, in such cases  Rav Shlomo Zalman Aurebach seems to allow for one to make the timer go on earlier in cases such as to care for someone ill or perform a mitzva, as long as there is still a delay before it turns on, so that it fulfills the condition of gramma. (Minchat Shlomo II, 27; Shmirat Shabbat k’Hilchata Chapter 13, notes 91, 93) Rav Shalom Masas (Shemesh u’Magen 3, 5) even allows one to make the timer go off earlier in cases of significant financial loss, as it is merely gramma it is allowed in such cases.

On Yom Tov

Tzitz Eliezer explains that Yom Tov allows for even more circumstances of gramma, therefore one may extend the current state on a Shabbat timer under any circumstance, and as long as there is even a minimal need, one may change the timer so it goes on earlier – such as to make the plata go on earlier to heat the food. (Tzitz Eliezer I, 20:5)