tevilah b'zmana mitzvah

rashi is mashma (13a 4 lines before the ens of skinny lines) that the mitzva is to become tahor as fast as possible. tosfos in beitza 18b clearly holds that the mitzvah applies only if one is going to the mikva and the 1st day possible. that is why he says there that today when the women are machmir and go to the mikva late (as a zava) they do not fulfil that mitzvah.