The Mishna in Brachos (20) lists certain mitzvos which women are exempt from, like tefillin, and others, like tefillah, which they are obligated to perform. The gemara explains that women are obligated in tefillah because Divine mercy is something that all humanity shares a need to pray for. According to Tosfos’ reading, the gemara then asks why the Mishna even needs to tell us that women are obligated in prayer – this principle is so obvious that it goes without saying. The gemara answers that had the Mishna not explicitly informed us that women were obligated one might have exempted them because prayer is performed at specific times of day – it is a mitzvah which is zman gerama, time delimited, from which women are usually exempt. The Mishna teaches that this case is an exeption.
Rashi emends the text to omit this question and answer. Women are only exempt from mitzvos d’oraysa, Biblical commandments, which are zman gerama. Prayer is only a Rabbinic obligation, and therefore, even if performed only at fixed times, women would not be exempt. Why did Rashi posit this distinction between Biblical and Rabbinic obligations with respect to the exemption of zman gerama? One possibility is that zman gerama exempts women from positive commandments, but not issurim, not prohibitions. The Torah itself prohibits violating the edicts of a Rabbinic court. Therefore, any legislation of the Rabbis, whether the enactment is a time-delimited obligation or not, must be obeyed by women because they have no dispensation from the Torah prohibition of ignoring Chazal.
As we have seen, the gemara gives the reason of af hein hayu b’oso hanes, that women initiated or were affected by the miracle, to explain why women are obligated to light a Chanukah menorah, drink 4 cups on Pesach, and read the Purim megillah. If not for that reason, women would be exempt from these mitzvos because they are time-delimited, zman gerama. This is very difficult to understand according to Rashi. Why does the gemara need af hein to explain why women are obligated in these cases - women are never exempt from any time-delimited Rabbinic enactments according to Rashi, and these cases should be no exception?!
I have one idea on this one so far, but the question is better than my answer. Any thoughts?