The gemara (Sukkah 28) quotes a derasha to teach that women are obligated in tosefes Yom haKippurim just as men are. The gemara explains the chiddush here is that since there is no punishment for violating tosefes you might have thought women are not chayavos, kah mashma lan that they are. So what if there is no punishment -- why should that lead one to think women are exempt? The Ritv"a explains that since there is no punishment it means tosefes does not have a lav associated with it like inuy on Yom Kippur proper does. Therefore, since it is only an aseh which is zeman gerama you would have thought women are exempt, kah mashma lan the pasuk that in this mitzvah women are chayavos.
There is, however, another way you can read the kah mashma lan here, but it needs a little background to make sense:
Tosfos (Kiddushin 29) asks why we need a pasuk to tell us that women are exempt from the mitzvah of milah -- isn't it a mitzvas aseh she'hazeman gerama, and we already know the rule that women are exempt from all mitzvos aseh she'hazeman gerama. The Tos RI"D gives an answer that, to be honest, can be interpreted in a few different ways depending on how you read the words. The way the Divrei Yechezkel siman 45) reads it as a chiddush is the definition of zeman gerama. Zeman gerama means a specific point in time serves as the cause of the mitzvah. The night of 15 Nissan is a specific point in time which generates an obligation to eat matzah. The 15th of Tishrei is a day that carries with it an obligation to eat in Sukkah. If I ask you to take out a calendar and tell me which day of the year generates the chiyuv of milah, you can't do it. The cause of the chiyuv of milah is the birth of the baby. Just because you can't do milah until eight days later does not mean that the time is the cause of the mitzvah -- all it means is that the kiyum mitzvah can only be done then. It's like saying you need a knife to do milah: that doesn't mean the knife is the cause of milah; all it means is practically you can't do milah without it.
The Ramban in his chiddushim to Kiddushin has a strange comment (and it may simply be a bad girsa) that sefiras ha'omer is not a mitzvas aseh she'hazeman gerama. How can that be -- the mitzvah is all about counting specific days between Pesach and Shavuos!? The Divrei Yechezkel suggests that the Ramban holds like the Tos RI"D. The Torah never says to start counting the omer on the 16th of Nissan -- sefirah is not caused by our reaching a specific day on the calendar. What the Torah says is that "m'macharas haShabbos," the day after Pesach, whatever day or date that may be, is a day you should start counting. It's like saying to do milah on the eighth day, whatever calendar date that may be.
Here too with respect to tosefes Yom haKippurim, one can read the gemara's conclusion not as kah mashma lan this is an aseh she'hazeman gerama and nonetheless women are mechuyavos, but rather as kah mashma lan tosefes is not a mitzvah she'hazeman gerama at all. There is no specific date or time that the Torah gives to do tosefes; it never says, for example, on the ninth of Tishrei and 5:30 accept upon yourself inuyim in advance. All the mitzvah says is that at some point before 10 Tishrei you need to have tosefes. Time is not the cause of the mitzvah; it's just another ingredient necessary to achieve the mitzvah's fulfillment.