Question: Can I use a timer on my hotplate to heat my chicken on Shabbos for lunch?
Answer: The Shulchan Aruch (OC 253:5) writes that one may heat up a davar gush, dry solid food on Shabbos by placing it on top of another pot. One may place such food onto a hotplate whether it is on or off (but will be switched on with a timer).
While one mustn’t place a davar lach, a boiled food with liquid on the stove or hotplate on Shabbos, R’ Ovadia Yosef (Yabia Omer 10 OC:26) allows one to place such a pot on the hotplate while it’s off even though it will later switch on through a timer.Most poskim, however, disagree. R’ Tzvi Pesach Frank (Har Tzvi OC 136) compares using a timer to later heat food to placing food on a stove that will be lit soon, which he argues is assur deoraisa. R’ Yehoshua Neuwirth (Shemiras Shabbos Kehilchasa 1:26), the Chazon Ish (38:2-3) and R’ Benzion Abba Shaul (Ohr Letzion 2:30:18) write that while it isn’t assur deoraisa, one still can’t do so on Shabbos because it is a problem of gerama, causing something to happen (though one could ask a non-Jew to do so while it was off).