Gemara

17a (Mishnah): If a Get was written during the day and signed at night, it is Pasul;

R. Shimon is Machshir.

18a (Shmuel): The Kesuvah is like an act of Beis Din. Just like acts of Beis Din may be written during the day and signed at night, also the Kesuvah.

The Kesuvah of Chiya, Rav's son, was written during the day and signed at night. Rav was present and did not say anything.

We may not infer that Rav agrees with Shmuel. They were engaged in the Kesuvah the entire time.

(Beraisa - R. Eliezer bar R. Tzadok): (If it was signed the night after it was written,) it is valid only if they were engaged in it the entire time.

(Reish Lakish): R. Shimon is Machshir the Get if it was signed that night, but not if it was signed (after this) during the next 10 days, lest he appeased her.

(R. Yochanan): R. Shimon is Machshir even if it was signed during the next 10 days. Had he appeased her, people would know about this.

(R. Yochanan): If Reuven told 10 people '(all of you) write a Get to my wife', he made only two witnesses, and merely stipulated that the others must sign on it;

(Reish Lakish): He intends that all 10 be witnesses.

A case occurred in which Reuven told 10 people 'write a Get to my wife.' Two of them signed that day, and the rest during the next 10 days. R. Yehoshua ben Levi said 'in pressed circumstances, we may rely on R. Shimon's opinion.'

R. Yehoshua ben Levi explains R. Shimon like R. Yochanan does (if witnesses sign within 10 days, it is valid), and holds like Reish Lakish (all 10 are witnesses).

86a (Mishnah): Three kinds of Gitin are Pasul, but if she remarried, the children are Kosher (not Mamzerim)... witnesses signed it, but there is no date on it.

R. Elazar says, even if no witnesses signed on it, but it was given in front of witnesses, it is valid. Witnesses sign a Get only for Tikun ha'Olam.

86b (Rav): If one remarried with one of these Gitin, she may stay with her new husband only if she has children from him. If not, she must leave.

(Levi and R. Yochanan): Even if she has no children, she need not leave.

Rishonim

Rif and Rosh (2:4): Shmuel allows a Kesuvah to be written during the day and signed at night. Perhaps Rav disagrees. He allowed this for his son's Kesuvah because they were engaged in it the entire time, like R. Eliezer bar R. Tzadok.

Rif and Rosh (46b and 9:6): If one remarried with one of these three Gitin, she may stay.

Ran (DH Gemara): Normally, one who remarried with a Pasul Get must leave. This applies to Pesulim particular to Gitin, lest people equate Gitin to documents. The three Pesulim of our Mishnah apply also to documents, so she need not leave. According to Rav, this is only if she has children.

Ran (DH v'Get): Some say that a predated Get is like one without a date. Some say that it is worse, lest she fraudulently collect Peros from one who bought from her husband before the divorce. Even according to Rashi, who says that buyers demand proof when she received it (not only when the Get was lost), perhaps she must leave, for also predated documents are Pesulim.

Rambam (Hilchos Gerushin 1:25): If a Get has no date, or it is predated or postdated, or it was written during the day and signed at night, it is Pasul. This is even if they were involved in it the entire time.

Rosh (2:9): Rav Hai Gaon says that some say that we may rely on R. Shimon in pressed circumstances only because two signed that day. It seems that in pressed circumstances, the Halachah follows R. Shimon in the Mishnah. They may sign within 10 days, like R. Yochanan, for R. Yehoshua ben Levi agrees.

Beis Yosef (EH 127 DH veha'Rosh): The Rif did not bring R. Yehoshua ben Levi's ruling. It seems that he does not rely on R. Shimon at all. Also the Rambam disqualified when it was signed the next night, without distinction.

Poskim

Shulchan Aruch (EH 127:1): If one was divorced with a Get without a date, she may not remarry. If she remarried, she need not leave, even if she has no children.

Beis Yosef (DH v'Chasuv): The Ba'al ha'Itur says that R. Elazar holds that if he gave to her a Get without a date, in front of witnesses, it is valid, and l'Chatchilah she may remarry. R. Elazar permits because witnesses sign only for Tikun ha'Olam. This implies that any Get lacking something merely for Tikun ha'Olam is Kosher. The Rashbam agrees. The Ramban refuted him, and also the Rambam and Tur say that it is Pasul.

Terumas ha'Deshen (19): A case occurred in which the year on a Get omitted the centuries, and she remarried. She needs another Get, but stays married. The Rambam calls such a Get Pasul; he does not say that it is not a Get.

Shulchan Aruch (2): If the Get is predated, or it was written during the day and signed at night, it is Pasul, even if they were involved in it the entire time.

Beis Yosef (DH v'Im): It was signed after that night, we are concerned lest they had Bi'ah (and people will say that the child was conceived out of wedlock - Rashi), or that he was Mevatel the Get (R. Tam).

Question (Beis Yosef DH v'Chosav Avi...): The Rambam disqualifies a predated Get (but she need not leave her new husband). The Rosh (46:5) holds that it is totally Batel, for the date is false. Why is this different than other Pesulim mid'Rabanan? The Terumas ha'Deshen (2:13) says that the Rosh equates it to a predated document, from which one may not collect at all (Tosfos Bava Metzi'a 72a DH Shtar). What is the source that Chachamim uprooted a Torah law, and if another was Mekadesh her, we ignore it?!

Answer (Bach DH Kosav): The latter Kidushin is a Ta'os (mistake). He would not be Mekadesh her if he knew that her Get is Pasul!

Rebuttal (Taz 2): Kidushei Ta'os is only when there was a stipulation that was not fulfilled. This case is like one who was Mekadesh, and found that she had a blemish. It is Safek Kidushin.

Beis Shmuel (2): If she married, she may stay, but she needs another Get. The Ran says that she must leave if she has no children. The Rosh says that she must leave in any case.

Shulchan Aruch (ibid.): Some say that if it was signed that night it is Kosher in pressed circumstances.

Beis Shmuel (3,4): The Rosh says so. The Rambam disqualifies, even though this is Kosher for monetary documents, for they have publicity. It is Kosher even if it was signed 10 days later, for we rely on R. Shimon, who is concerned only for Peros (but not for fraudulently exempting one who was Mezanah from Misas Beis Din). If the husband did not know that it was predated, all say that she must leave if she remarried.

Gra (3): The Rambam holds that R. Eliezer bar R. Tzadok permits this only for monetary documents. Tosfos and the Rosh hold that he discusses Gitin.

Rema: Even though a predated Get is Pasul, if someone else was Mekadesh her, she needs a Get from him and may not return to her first husband.