2 Hearos about z'man grama:
1. The gemara gives a list of mitzvos that are time bound so that women are exempt, and a list of mitzvos that aren't time bound so that women are obligated. Tosafos asks that in the list of mitzvos that are not time bound and woman are obligated, there are mitzvos that have a supporting lo ta'aseh, which would obligate them with or without the aseh. So, why is it important to list these as not being time bound so that women are obligated, since women are anyway obligated due to the lo ta'aseh? Tosafos answers that there are situations where the aseh applies without the lo ta'aseh, so it is important to say that women are obligated in the aseh even under those circumstances.
Tosafos offers another answer that if a woman would be obligated only in the lo ta'aseh then another mitzvas aseh could sometimes push off the lo ta'aseh, but since she is obligated in the aseh also, another mitzvah could not push it off (since aseh can't push off aseh and lo ta'aseh). Tosafos rejects this approach because even if the aseh is time bound, once they are obligated in a lo ta'aseh that has an aseh supporting it, the lo ta'aseh is strengthened so that another aseh cannot push it off.
The Ran assumes that whenever there is a lo ta'aseh together with an aseh, even if the aseh is time bound, women are obligated in the aseh and lo ta'aseh. But this is only if the aseh and lo ta'aseh are inseparable, so that whenever the aseh is present the lo ta'aseh is also present. But if the aseh is sometimes separated from the lo ta'aseh then being obligated in the lo ta'aseh would not obligate them in the aseh.
R' Akiva Eiger points out that Tosafos and the Ran argue when you have a lo ta'seh and aseh that is time bound, Tosafos holds that women are only obligated in the lo ta'aseh, but the Ran holds that they are also obligated in the aseh.
2. Tosafos assumes that women are not obligated in the aseh of yom tov since it is time bound. R' Akiva Eiger says that when we exempt women from time bound mitzvos, the mitzvos must be similar to tefillin which are fulfilled b'kum v'aseh (actively), but mitzvos that are fulfilled passively, even if they are time bound women are obligated in. R' Akiva Eiger says that if a woman misses ya'aleh v'yavo on yom tov, according to Tosafos she doesn't have to repeat. Why? Because she is patur from the mitzvah of simchas yom tov, therefore is allowed to fast, and therefore doesn't repeat birchas hamazon (since the gemara says that repeating doesn't apply to cases where אי בעי אכיל אי בעי לא אכיל). But according to R' Akiva Eiger's suggestion that a passive mitzvah women are obligated in even if it is time bound, she would have to repeat birchas hamazon.
The Minchas Chinuch (112) clearly disagrees with R' Akiva Eiger. The Minchas Chinuch is troubled with why the poskim seem to assume that a woman is obligated in the mitzvah of shevisas ha'aretz during shemittah - it is a time bound positive mitzvah? The Minchas Chinuch explains that the rule of time bound positive mitzvos is only an exemption for mitzvos on one's body but is not an exemption for mitzvos outside of one's body (with this he answers Tosafos question 29a that time bound positive mitzvah would not be sufficient to exempt a mother from milah on her son since it is a mitzvah not on her body). Being that the mitzvah to rest the land during the shemittah year is not a mitzvah on her body, the exemption of z'man grama doesn't apply so she is chayev. Based on R' Akiva Eiger, it is much easier to explain why she is chayev, it is a passive mitzvah - from the fact that the minchas chinuch needs to jump through hoops to explain why a woman is chayev in shemittah (at least plowing during shemittah which is only an aseh), he clearly rejects the sevara of R' akiva eiger.