Gemara

Question: Why was it enacted to put the date on a Get?

Answer #1 (R. Yochanan): We are concerned for a man who married his niece. (He might have compassion on her if she was Mezanah (had Bi'ah with another man), and save her from execution by giving her a dateless Get. Beis Din would have a Safek if the Bi'ah was before or after the divorce, and would not kill her.)

Answer #2 (Reish Lakish): The date enables her to prove from when the Peros (produce) of her property belong to her, i.e. the day he finished writing the Get.

Reish Lakish didn't answer like R. Yochanan, because Zenus is not common. (Chachamim decree only about what is common.) R. Yochanan didn't answer like Reish Lakish, for he holds that the husband eats the Peros until the Get is given.

Contradiction: Elsewhere, R. Yochanan taught that a wife receives the Peros of her property from the day her husband writes a Get, and Reish Lakish said that the Peros are hers from the day he gives it!

Answer: We must switch the opinions.

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.

17b (Abaye) Question: Since a Get without a date is Kosher b'Di'eved, what did the enactment to put a date on the Get accomplish?

Answer (Rav Yosef): L'Chatchilah, she may not remarry with such a Get.

Question: If he will cut off the date and give her the Get, the enactment does not help!

Answer: We are not concerned for such swindlers.

26b - Question: Why does a Get from Eirusin need a date? Granted, there is concern for saving her from being killed, but how will the other opinion answer? A husband has no rights to Peros before Nisu'in!

Answer (Rav Amram citing Ula): It is an enactment on behalf of the child (lest he give it after Nisu'in and people think that she conceived from him after divorce).

Yevamos 52a (Rami bar Chama): If a man asked a scribe to write a Get to his Arusah to give it after Nisu'in, the Get is valid, since he could divorce her now. If he authorized a Get before Eirusin, it is invalid, since he cannot divorce her now.

Rishonim

Rif and Rosh (2:4): Some say that the Halachah follows Reish Lakish, just like we count the three months (that a divorcee must wait before remarrying, to avoid doubts about who fathered her child) from the writing. This is wrong. They are not secluded from the writing, (so there is no concern for Bi'ah, so) we may count the three months from then. A husband eats the Peros in exchange for his obligation to redeem her if she will be captured. Since he is obligated until she gets the Get, he eats the Peros until then. This shows that the Halachah follows R. Yochanan.

Rosh: Even nowadays that there is no Misas Beis Din, a Get without a date is Pasul, like the Mishnah (86a), which Rebbi codified. There is concern lest he cover up for her children (from Zenus), who are Mamzerim. Or, perhaps he does not want her to become forbidden to him through Zenus before divorce. I disagree. If it were appropriate to obligate a date for these reasons, R. Yochanan would not say that it is to save one's niece from death. These reasons apply to any wife! One does not want rumors about one's child that he is a Mamzer, nor that one's wife will be forbidden to him. Perhaps these are supporting reasons, and R. Yochanan mentioned the primary reason. Perhaps nowadays, even though the primary reason is Batel, we still disqualify due to the other reasons.

Rosh (ibid.): R. Chananel said that we switch the opinions in the latter teachings to match the opinions above (R. Yochanan is concerned for saving from death). The Ba'al ha'Itur says that in the Yerushalmi, R. Yochanan is concerned for Peros, and Reish Lakish is concerned for saving from death! Rather, we switch the opinions above. We hold like R. Yochanan, that the concern is for Peros. Even if we switch the latter teachings, we rule that the concern is for Peros. We asked about (26b) why an Arusah's Get needs a date according to the opinion that is concerned for Peros, and Ula answered. This shows that he is concerned for Peros. Rami bar Chama (Yevamos 52a) said that if a man asked a scribe to write a Get to his Arusah to give it after Nisu'in, it is valid. This shows that he is not concerned for saving from death. We do not follow R. Yochanan against a majority, i.e. Reish Lakish, Ula and Rami bar Chama.

Rebuttal (Rosh): There is no proof from Rami bar Chama! His main Chidush was that a command to write a Get before Eirusin is invalid. All agree that l'Chatchilah one may not command a Get before Nisu'in to give after Nisu'in, either due to saving from death, or precluding suspicion about the child. There is no proof how Ula holds. Amora'im often answer on behalf of an opinion they argue with. There is no proof from the Yerushalmi. It argues with the Bavli. It does not mention concern for saving from death. The Bavli said 'we switch the opinions' without saying 'Ela' (rather). This shows that we switch the latter teachings. Therefore, we hold like R. Yochanan, that we did not enact a date for the sake of Peros.

Rambam (Hilchos Gerushin 1:24): Chachamim enacted to write the date in a Get lest his wife is his relative, she will be Mezanah while married, and he will write a Get for her afterwards. If it will have no date, she can say it preceded the Zenus.

Rambam (25): If a Get has no date, it is Pasul.

Rambam (26): If he cut off the date and gave it to her, it is Kosher.

Poskim

Shulchan Aruch (EH 127:1): One must write the date in a Get. If one was divorced with a Get without a date, she may not remarry.

Beis Yosef (DH Tzarich): The Rif, Rosh and Rashba refuted the opinion that the enactment was for the sake of Peros. Also the Rambam says that it was lest one save his wife from death.

Beis Yosef (DH Aval Kivan): The Tur says that a Get without a date is Pasul only mid'Rabanan. Therefore, if she was Mezanah and can prove that she received the Get beforehand, she is not Chayav Misah, the children are Kesherim and she is permitted to her husband.

Bach (DH Aval): The Beis Yosef does not mean that we would kill her if she could not verify when she was divorced. Rather, he explains why the date was enacted. She must verify that she was divorced first to be Machshir her children and to be permitted to her husband.

Rebuttal (Taz 1): If so, the enactment did not help. He can still save her from death! Rather, since she may not remarry l'Chatchilah, if there is no date it is clear that he did not write the Get to permit her to marry, rather, to save her from death. We kill her. However, if the date was cut off, we do not kill her, for perhaps it was cut off inadvertently, of someone else cut it off.

Note: We hold that one is killed only if he was warned and said 'I sin, knowing that I will be killed for this' (Sanhedrin 40b, Rambam Hilchos Sanhedrin 12:2). Why would she say so if she already was divorced?! Perhaps she jested (the Bo'el will enjoy it more if he thinks that she is forbidden - Nedarim 91b), for she was sure that she could exempt herself (she assumed that the Get had a date, or that she could find the Edei Mesirah). Or, she will say that only later she found out that her Shali'ach had already received her Get at the time.