Women are exempt from any Mitzvah d"Oraisa that is Zeman Grama, tied to a date. They should therefore be exempt from Megilah, if not for the fact that they had a part in the miracle.

The rule that women are exempt from Mitzvas Aseh sheha'Zeman Grama applies to Mitzvos d'Rabanan as well; see Tosfos Berachos 20b DH bi'Tefilah.

Be well,

Mordecai Kornfeld

Your question is asked by the Gemara, BERACHOS 20b. The Mishnah that you cited states that women are obligated in Tefilah. The Gemara states that the reason is because prayer is "Rachami" i.e. one prays for compassion from Hash-m. The Gemara says that one might have thought that since Tehillim 55:18 says that I pray "evening, morning and afternoon" this might therefore be considered as a Mitzvas Aseh sheha'Zman Grama so women should be exempt - therefore it is necessary for the Mishnah to say that they are obligated because women also need to request mercy from Hash-m.

That is the simple answer which is true according to our text in the Gemara. However it is not so simple (it never is!) because there are different texts (this is already apparent from Rashi and BACH) and in fact RIF (11a in his pages) writes that Tefilah is in fact a Mitzvah not dependent on time. RABEINU YONAH DH GM' TEFILAH gives 2 reasons for this.

1. Even though prayer is dependent on time, nevertheless since the Gemara says "If only a person would pray all day long", it is therefore considered a Mitzvah not dependent on time

2. Because Tefilah is "Rachami", it is therefore considered as not dependent on time. RAMBAM HILCHOS TEFILAH 1:2 also writes that Tefilah is a Mitzvah not dependent on time. (See also BACH on TUR OC end 106 and PRI MEGADIM MISHBETZOS ZAHAV 106:2.)

May we be Zocheh to a Ge'ulah Shlemah and Binyan Beis ha'Mikdash.

KOL TUV

D. Bloom