I noticed the Netziv at the end of Chayei Sarah writes that the Imahos did not keep mitzvos aseh she’hazman gerama. I never really thought about it before, but why should this be so? The fact that women are not commanded to do these mitzvos is irrelevant because the Avos and Imahos had not been commanded to do any mitzvos.
Let me put it this way – is a woman who voluntarily does a zman gerama mitzvah, e.g. she listens to shofar, fulfilling the ratzon Hashem? If the answer is no, then why can she say a bracha and get schar, and if the answer is yes, then why would the Imahos have not done these mitzvos?