"Vehayah Lifnos Erev Yirchatz ba'Mayim". What is the significance of the words "Lif'nos Erev ... "?
Rashi: One needs to Tovel shortly before nightfall, because after Toveling, one only becomes completely Tahor at nightfall. 1
Sifri: It implies that, if someone sees Zivus within twenty-four hour after Keri does not become a Zav.
See Sifsei Chachamim.
Is it an obligation to Tovel before nightfall?
Rashi (in Shabbos, 121a) #1: 'Yes!' R. Yossi learns from here that it is a Mitzvah to Tovel when the time to Tovel falls due - 'Tevilah bi'Zemanah Mitzvah'.
Rashi (in Shabbos, 121a) #2: 'No!' TheTorah is merely giving the Ba'al Keri sound advice - because if he does not Tovel before nightfall, he will not become Tahor until the following night.
