Gemara

29a (Mishnah): Women are exempt from a Mitzvas Aseh sheha'Zman Gerama (it does not apply at all times), and obligated in an Aseh she'Lo ha'Zman Gerama.

33b (Beraisa): The following Mitzvos are not Zman Gerama: Ma'akah (building a railing around one's roof), Hashavas Aveidah (returning a lost object), and Shilu'ach ha'Kan (sending the mother bird).

Question: The Mishnah's law is not always true! Simchah (on Yom Tov) and Hakhel (going to the Mikdash on Sukos to hear the Torah) are Zman Gerama, yet women are obligated! Learning Torah, Peru u'Rvu (having children), and redemption of a firstborn son are not Zman Gerama, yet women are exempt!

Answer (R. Yochanan): We do not learn from general rules, even when the rule lists exceptions.

Rishonim

Rambam (Hilchos Avodah Zarah 12:3): Women are exempt from all Mitzvos Aseh that come from time to time and are not constant, except for Kidush ha'Yom, eating Matzah on the (Seder) nights of Pesach, eating and Shechitah of Korban Pesach, Hakhel and Simchah.

Question #1 (Ran 14b DH v'Ei Zo): The Beraisa obligates women in an Aseh she'Lo ha'Zman Gerama such as Ma'akah, Hashavas Aveidah and Shilu'ach ha'Kan. This implies that if these were Zman Gerama, women would be exempt. Each of these has a Lav. Since women are obligated in all Lavin, they are also obligated in the Aseh!

Question #2 (Tosfos Kidushin 34a DH Ma'akah): It does not matter whether or not the Aseh applies to women, since in any case they must do the Aseh to avoid transgressing the Lav!

Answer #1 (to both questions - Ri, in Tosfos, ibid.): In each case the Aseh can apply without the Lav. If one built a house, or picked up an Aveidah or mother bird with intent to fulfill the Aseh, and then reconsidered, he transgresses the Aseh, but not the Lav.

Answer #2 (to Question #1 - Ran, ibid.): We say that when a women is obligated in a Lav, she is obligated in the Aseh, only if it is impossible to transgress one without the other. An example is resting on Yom Tov and not doing Melachah on Yom Tov.

Answer #2 (to Question #2, brought in Tosfos, ibid.): In any case we need to know that the Aseh applies to them. If only the Lav applied to them, a different Aseh could override the Lav (e.g. if she needs to offer a bird for Korban Yoledes and the only bird she can find is sitting on eggs).

Answer #3 (to both questions - Ritva 34a DH v'Rabbeinu, citing the Ramban): In these Mitzvos, the Torah's primary concern is the Aseh. If women were exempt from the Aseh, they would be exempt also from the Lav!

Ha'Makneh (on this Tosfos, DH b'Emtza): If an Aseh entails an action, the Lav is merely to strengthen it. If the Aseh is fulfilled through inaction, the Lav is primary. The Aseh merely strengthens the Lav, so women are commanded also in the Aseh.

Question (R. Yosef of Eretz Yisrael): The Aseh to burn Pasul Kodshim does not override the Lav and Aseh of Melachah on Yom Tov (Shabbos 25a). For women, who are commanded only about the Lav, the Aseh should override the Lav!

Answer #1 (R. Yosef in Tosfos, ibid.): Since there is an Aseh with the Lav, even though women are exempt from the Aseh, this shows that the Lav is more stringent, so an Aseh does not override it. (This refutes Answer #2 above (6).)

Answer #2 (Ran, ibid.): Because the Lav applies to women, also the Aseh does.

Answer #3 (Pnei Yehoshua 34a DH b'Emtza): The Mitzvah to burn Pasul Kodshim applies only during the day, so women are exempt. R. Yosef refers to burning Tamei Terumah (oil). It surely applies at night, for we expound that one may benefit from it, i.e. the light, while it burns. Just like it is a Mitzvah to burn Tamei Kodshim, it is a Mitzvah to burn Tamei Terumah (Shabbos 25a). R. Yosef holds that it is mid'Oraisa. His question is not difficult for the opinion in Tosfos (Shabbos 25a DH Kach) that it is a Mitzvah mid'Rabanan.

Note: It is not clear how this proves that the Mitzvah applies at night. One can use the oil during the day to cook, heat, or illuminate rooms without windows. Or, perhaps one may use the light at night, but he must light the fire during the day!

Answer #4 (ha'Makneh, ibid.): We learn that women are exempt from Mitzvas Aseh sheha'Zman Gerama from Tefilin. We learn only similar Mitzvos, in which there is no Lav. ("Hishamer" denotes a Lav, but when it is written about an Aseh, such as Tefilin, it is an Aseh - Eruvin 96a). When there is a Lav, women are commanded in both. We needed a Hekesh for Shamor and Zachor, for they are different Mitzvos. The Aseh and Lav of Melachah on Yom Tov are the same matter, so both apply to women. If one returns a loan in Shemitah, the lender must say 'I am Meshamet'. Surely, women are not exempt! Since there is a Lav with it, "Lo Yigos", we cannot learn from Tefilin to exempt women. Perhaps Shemitas Kesafim is not Zman Gerama, for the loan is permanently cancelled. However, Tosfos (DH Tefilin) says that Tzitzis is considered Zman Gerama because the time we wear the garment determines whether or not it is obligated. If so, also Shemitas Kesafim is Zman Gerama!

Einayim l'Mishpat (Kidushin 34a Tosfos DH Ishah): The Rambam obligates women in the Aseh of Yom Tov (Sefer ha'Mitzvos explicitly says whenever women are exempt), even though this is not explicit in the a Mishnah Torah.

Question (Sha'ar ha'Melech Avodah Zarah 12:3): We expounded a verse (Yevamos 6a) to teach that Kibud Av va'Em does not override Shabbos. How did the Gemara try to infer that an Aseh does not override a Lav? There is also an Aseh to rest on Shabbos! According to Tosfos, we can say that the verse is needed for women, who are exempt from the Aseh. However, why don't we say that since the Lav applies to women, also the Aseh?

Question (Sha'ar ha'Melech, ibid.): The Ran holds that if one cannot transgress the Aseh without the Lav, they depend on each other. This can explain why the Rambam and Sefer ha'Chinuch exempt women from the Lav of entering the Mikdash without a Korban. "Zechurcha" exempts them from the Aseh of Re'iyah. R. Yosef in Tosfos holds that Aseh and the Lav never depend on each other. Why are women exempt from the Lav? If the Lav applies to them, this should have been taught somewhere. Perhaps "Zechurcha" also excludes women from the Lav. However, the Gemara asked why Zechurcha is needed to exclude women from the Aseh. If it also teaches about the Lav, there is no question (and there is a better answer)!

Answer (Ma'aseh Choshev 30): Here, obviously women are exempt, since only one who is obligated may bring a Korban on Yom Tov.

Note: The Lav applies to Yom Tov and days of compensation. When the first day (Yom Tov) is on Shabbos, also men cannot bring that day! Beis Shamai never permit Olas Re'iyah on Yom Tov (Beitzah 19a)! This Lav is not listed among the three Lavim from which women are exempt (Kidushin 29a). We do not learn from Kelalim. However, we rule unlike Isi (who exempts women from Karachah) because our Tana did not mention this (Ran 15a DH Isi).