Why are timers allowed to be used on Shabbos?
We don’t cut nails on Thursday because of the resulting growth of the nails that will start on Shabbos. Why are timers allowed to be used to make appliances start running on Shabbos? Is there a difference?
The reason why some are particular — and this is this the common custom —not to cut nails on a Thursday is because they will grow back on Shabbos. This will not be considered respectful for Shabbos as they grow back on Shabbos and hence lacking the mitzva of כבוד שבת. The theory that the reason for the custom is because it’s a סרך מלאכה, looks like a melacha, has been rejected by most poskim. Either way it’s clearly not an issur and the custom developed only in relation to cutting nails.
As for timers, most contemporary poskim maintain that one may use times even though they will be starting a melacha on Shabbos. Some argue that this is considered a violation of the general principle of resting on Shabbos, since many businesses nowadays can be run on pre arranged timers or that it considered degrading to Shabbos to have such a set up. They consider it somewhat akin to the rabbinic prohibition against having a non-Jew work for us on Shabbos, since it might result in Shabbos becoming lenient in one’s eyes and causing him to violate Shabbos himself, Heaven forbid.
Nonetheless, the general custom is to be lenient in this matter. Certainly in a place where not having a timer could cause more widespread חילול שבת, one should affix a timer to avoid such potential problems.
For more info about timers, see the references here:
בטעם המנהג – ראה שו״ת מהרי״ל החדשות רד. אדני פז רס, א. שו״ת בית יעקב מח. א״ר רס, ה. והביאו בפמ״ג שם במ״ז א. ערוה״ש שם ו. שבה״ל ו, כא, ב.
ושו״ר שהעיר מזה בשו״ת ארץ צבי א, קט ד״ה עוד יש לדון. ולפלא שנעלם ממנו כל הנ״ל.