The Mishna in Kiddushin tells us that women are generally exempt from mitzvos aseh she’hazman gerama, mitzvos which are limited to being performed only at certain times. There are numerous cases where at first glance one would have though a mitzvah is zman gerama, time-delimited, but the gemara in fact calls on another reason to exclude women or women are in fact included in the mitzvah. Rishonim and Achronim offer various explanations for these exceptions, some focusing on unique attributes of particular mitzvos, some focusing on honing the definition of what exactly zman gerama means. There are so many answers to these type questions that the answers themselves serve as a catalog of the different darchei halimud, the thinking and methodology of those who proposed them.
Although there is a mitzvah to give testimony, the gemara (Shavuos 30) derives from the word ‘anashim’ that only men and not women may serve as witnesses. The question is raised: since din, the judgment of court cases, may not take place at night, testimony may also not be given at night. If so, why should we require a separate source derasha to exclude women from being witnesses – isn’t this a time bound commandment?
My chavrusa suggested that although one cannot testify except by day, the obligation to testify is ongoing – a limitation on the kiyum mitzvah but not the chiyuv is not a zman gerama type mitzvah. I would put it more succinctly – the limitation of night vs. day is a halacha in bais din, not in eidus – it defines the parameters of when court sessions take place, not the parameters of the obligation to testify. My chavrusa and myself have been too influenced by standard Litvishe lomdus to even dream of a sevara like that suggested like the Maishiv Davar: since eidus is a form of chessed that is done on behalf of one of the parties, and women are obligated in chessed, one would have thought even women are obligated to give testimony if not for a specific exclusion. Different ways of thinking – different answers, and many others are possible as well (feel free to add your own in the comments!).
Another example from our parsha: the Torah says that if a slave wishes to remain with his master after his 6 years of labor are complete, he must have his ear pierced next to a doorway – the mitzvah of retziya. As Rashi explains, the slave’s ear which heard on Sinai that a Jew should be a slave only to G-d but not to his fellow man is pierced because this person has chosen to remain a servant rather than accept the gift of freedom. The halacha is that retziya may only be done at day and not at night. Nonetheless, the halacha of retziya applies to all slaves, even a slave owned by a woman. Why would the mitzvah of retziyah not be counted as a mitzvah which is zman gerama, time dependent, which would exempt a woman owner from piercing the ear of her slave? Again, many answers possible, but no hints from me on this one...