Gemara

(Beraisa - Beis Hillel): "L'Vas" includes a (second Korban for a) miscarriage on night 81 (after giving birth to a girl, for on day 81 she could have brought her Korban). If a man has three emissions of Zivah within three days, he must bring a Korban after becoming Tahor, no earlier than eight days from the last emission.)

Question (R. Oshaya): If a Zav (liable to bring a Korban) had three more sightings on the eighth night, what is the law?

Suggestion: Tana'im argue about this;

(Beraisa #1): If a Zav had three sightings on the eighth night, he brings a (second) Korban.

(Beraisa #2): He does not bring a Korban.

Rejection (Rav Huna bar Acha): No, both hold that night is Mechusar Zeman. (It is as if it is not yet time to bring the Korban.) Beraisa #1 discusses a Zav not yet obligated to bring a Korban. Beraisa #2 exempts from a second Korban one who was already Chayav Korban.

(R. Yochanan): If a Zav had one sighting on night eight and two more on day eight, he brings a (second) Korban. If he had two sightings on night eight and one more on day eight, he is exempt.

Question: R. Yochanan holds (elsewhere) that night is Mechusar Zeman!

(Chizkiyah): If a Nazir (Tamei became Tahor, and again) became Tamei on the eighth day, he brings another Korban. If he became Tamei on the eighth night, he does not bring another Korban;

(R. Yochanan): Even if he became Tamei on the eighth night, he brings another Korban.

Answer: Above, R. Yochanan taught according to the opinion that night is Mechusar Zeman. (He himself disagrees.)

Zevachim 12a - Contradiction (R. Aftoriki): "V'Hayah Shivas Yamim Tachas Imo" implies that on the night before its eighth day it is Kosher for a Korban;

"Umi'Yom ha'Shemini va'Hal'ah Yeratzeh" implies that the night before its eighth day it is not Nir'eh!

Answer (R. Aftoriki): The night before it is Kosher to be Hukdash. It is not Kosher to be offered until the eighth day.

Yevamos 71a - Question (Rav Chama bar Ukva): May we anoint an Arel (uncircumcised) baby with Terumah oil?

Does Orlah (foreskin) forbid one to eat Terumah even before the proper time for circumcision (the eighth day)?

Answer (R. Zeira - Beraisa): The Torah taught that failure to circumcise one's son forbids him to offer Korban Pesach. A Gezeirah Shavah teaches that this forbids also eating Korban Pesach.

Question: How can one have an Arel son at night, at the time we eat Korban Pesach, but he did not have the son when we offer it (in the afternoon)?

Suggestion: A son was born in between. This shows that Orlah forbids even before its time!

Rejection (Rava): "He will circumcise every male, then he will bring," but before its time, he may not circumcise!

Rather, his father was imprisoned at the time of Hakravah, and was released before the time of eating.

Shabbos 136a: Rav Papa and Rav Huna brei d'Rav Yehoshua visited Rav Idi bar Avin. He prepared for them a calf on its seventh day. They told him 'had you waited until evening to slaughter it, we would have eaten it. Since you slaughtered it now, we will not eat it' (it is a Safek Nefel).

Rishonim

Rambam (Hilchos Mechusarei Kaparah 3:4): If a Zav had three emissions, and after seven clean days, on the night before he is due to bring a Korban, he had three more emissions, he does not bring another Korban for them, since they were before the time to bring the Korban for the first ones. Night is Mechusar Zeman for everything, except for a Yoledes who miscarried on the night before 81 days after giving birth to a girl. A tradition teaches that she brings another Korban for the miscarriage.

Kesef Mishneh: This is difficult. The Gemara (Kerisus 8a) concluded that R. Yochanan holds that night is not Mechusar Zeman! We can say that the Rambam rules like Chizkiyah, who says that night is Mechusar Zeman. The Gemara brought Tana'im who hold like this.

Rambam (Hilchos Chagigah 2:5): If one became Tamei on the first night of Yom Tov, even though he became Tahor the next day he is exempt, because all days are compensation for the first day. If one became Tamei on the first day of Yom Tov, he must bring Chagigah and Olas Re'iyah when he becomes Tahor.

Lechem Mishneh: The Rambam rules here and in Hilchos Mechusarei Kaparah that night is Mechusar Zeman. The Kesef Mishneh gave a reason. Also, the Halachah follows Chizkiyah against his Talmid R. Yochanan. Therefore, it is as if he was Tamei before the festival began. According to the opinion that night is not Mechusar Zeman, he is obligated.

Rambam (Hilchos Terumos 11:7): We may anoint a baby (male Kohen) with Terumah oil within seven days, for all seven days he is not considered an Arel (uncircumcised).

Aruch ha'Shulchan (YD 262:4): The Yerushalmi says that Orlah is only from the eighth day and onwards, but not before. One may anoint the baby with Terumah before this, even though one may not anoint an Arel with Terumah. The Bavli (Yevamos 71a) said that Orlah before he can be circumcised is not called Orlah.

Avnei Nezer (YD 3): The Rambam rules that Orlah before the time is not Orlah, like the Yerushalmi. The Bavli did not resolve this. The Kesef Mishneh said that it was resolved, but this is astounding. The Gemara said so only regarding the father's Heter to offer Korban Pesach. We cannot derive whether the baby is considered an Arel. The Rambam connotes that one may not anoint the baby on the eighth night. The Bavli holds that if Orlah within the time is not Orlah, i.e. before he may be circumcised, the same applies to the eighth night. If it were considered Orlah, why was it difficult to find an Arel at the time of eating, who was not Arel at the time of Hakravah? Perhaps the eighth night was Pesach night! Rather, the Rambam learns from the Yerushalmi, which forbids the eighth night.

Rashi (Zevachim 12a DH li'Kedushah) Mechusar Zeman does not apply to the night before, since we can offer it the next day.

Shitah Mekubetzes (12a:8): If one was Makdish an animal before the eighth night, it is not Kodesh. We find that a fetus can be Kodesh, i.e. along with its mother, but not if one was Makdish it by itself. Alternatively, the Torah disqualified Mechusar Zeman only "Asher Yivaled", i.e. after birth, when it is feasible to offer it.

Tosfos (Temurah 19b DH Af): In Bechoros (21b) we suggested that we should learn from Kodshim, in which Mechusar Zeman does not become Kodesh! We say (e.g. Zevachim 12a) that one may be Mekadesh a Korban on the night before the eighth day, and offer it the next day. This connotes that one cannot be Makdish it before this! We can say that this is l'Chatchilah, but b'Di'eved if one was Mekadesh it even earlier it is Kodesh, for we learn from Bechor.

Poskim

Shulchan Aruch (18:1): The Rambam holds that anything one wears at night is exempt, even if the garment is normally worn during the day.

Mishbetzos Zahav: If Tzitzis of a day garment snapped, one may make Tzitzis for it at night? This is not Ta'aseh v'Lo Min ha'Asuy. Even though the Rambam exempts a day garment at night, we find that Mechusar Zeman does not apply to a Mitzvah that will apply later that day.

Yabi'a Omer (8 OC 3): Nachalas Binyamin (12) and Pesach ha'Devir (1:18) asked why the Pri Megadim needed to learn from Ein Mechusar Zeman for the same day. We can learn from OC 626:3, in which the Rema rules like Ri ha'Lavan, that Ta'aseh v'Lo Min ha'Asuy regarding Schach is only when the Pesul is in the Schach itself! (Similarly, it applies only to a Pesul in the Tzitzis themselves.) I answer that the Pri Megadim teaches the law that Mechusar Zeman does not apply to the same day. Rashi (Zevachim 12a) says that Mechusar Zeman does not apply to the night before, since we can offer it the next day. The Shitah Mekubetzes says that if one was Makdish it within seven days, it is not Kodesh. Tosfos (Temurah 19b) says that b'Di'eved, it is Kodesh. However, the Rambam (Hilchos Mechusarei Kaparah 3:4) rules that night is Mechusar Zeman. Why was it clear to the Pri Megadim that night is not Mechusar Zeman, unlike the Rambam?! Maharsham (OC 18) says that the Rambam contradicts what he wrote in Hilchos Terumos.

Shulchan Aruch (YD 15:2): If we are unsure whether an animal had a full gestation, it is a Safek Nefel until the eighth night begins.

R. Akiva Eiger: The Taz (2, b'Sof) connotes that we need seven 24-hour periods. We do not say that part of the day of birth is like the entire day. The Pri Megadim agrees. Tevu'os Shor is lenient. I agree.

Taz (3): Even though regarding Korban, one must wait until the eighth day, that is because we do not offer Korbanos at night.

Gra: We learn from Shabbos 136a.