Parshas Terumah: Summary of a shiur by Rav Yosef Greenwald
The Rambam (Hilchos Bais HaBechirah 1:12) writes that the mitzvah of building the Bais HaMikdash applies only by day. He also rules that women are obligated in this mitzvah.
If the mitzvah only applies by day, it is a mitzvas asei shehazman grama, a time-bound positive mitzvah, from which women are patur. So why are they chayav here?
The Bais Yitzchak (from Turei Even, Chagiga 16b)answers that the mere act of building does not comprise the mitzvah of Binyan Bais Habechirah. Rather, the mitzvah is to have a Bais Hamikdash by means of constructing one. Because the mitzvah is the end state, once the Bais Hamikdash is built, the mitzvah is fulfilled in perpetuity as long as the building stands. And because the mitzvah fulfillment is not limited to daytime, this is not reckoned a zman grama.
The Maharach Ohr Zarua identifies a similar idea in bris milah, where the Gemara (Kiddushin 29b) derives from a pasuk that women are patur. Why is a pasuk necessary? Isn’t bris milah a mitzvas asei shehazman grama? He answers that the ikar mitzvah is for the child to be mahul forever, and that’s not called zman grama. (Tosafos there disagrees.)
The Minchas Chinuchfinds the same concept in shemita, where women are chayav even though it would appear to be zman grama, because the primary mitzvah is to have the land rest (chovas karka).
Another answer to our original question comes from Har HaMoriya: True, the mitzva is by day, but preparations and tirdos apply at night, too, so the mitzvah is not zman grama. The Tosfos Rid makes a similar point about bris milah.