Gittin 17B

זנות לא שכיחא.

Adultery is infrequent, and the rabbis would not institute the dating of a bill of divorce to avoid an infrequent problem.

ורבי יוחנן, מאי טעמא לא אמר כריש לקיש? קסבר: יש לבעל פירות עד שעת נתינה.

The Talmud asks: And what is the reason that R' Yoḥanan did not say in accordance with the reason of Reish Lakish? The Talmud answers that he holds the produce belongs to the husband until the time of the giving of the bill of divorce, and only afterward does the woman have the rights to the produce of her property. If she attempts to collect the value of the produce sold after her divorce, she will be asked to prove when she received the bill of divorce. Therefore, in terms of assisting her to collect these monies, the dating of the bill of divorce does not serve any purpose.

בשלמא לריש לקיש, משום הכי קא מכשיר רבי שמעון. אלא לרבי יוחנן, מאי טעמא דרבי שמעון ⁠– דמכשיר?

The Talmud continues and asks: Granted, according to Reish Lakish, due to that reason R' Shimon deems valid a bill of divorce that was signed on the day after it was written, because he holds that the rights to the produce of usufruct property revert to the woman the moment the bill of divorce is written. She is therefore within her rights to collect these monies from the date written in the bill of divorce, even if it was given at a later date. However, according to R' Yoḥanan, who holds that the reason for writing the date is to prevent the husband from shielding his wife from punishment for her infidelity, what is the reason that R' Shimon deems it valid? There is still a concern that he will have the bill of divorce written and dated earlier in order to protect her.