Gittin 31B

מעת לעת של הנחה.

It means from the time that it was set aside until the same time the day after it was set aside. In other words, he must be concerned about all of the untithed produce that he had rendered fit after the first 24 hour period from when he set aside the produce or money had passed.

תנן: אם אבדו ⁠– הרי זה חושש מעת לעת. בשלמא למאן דאמר מעת לעת של בדיקה ⁠– שפיר;

We learned in the Mishnah: If he discovers that they were lost, then he must be concerned that the produce or money that he set aside was lost from the time until the same time. Granted, according to the one who said that this means from the time it was checked and found to be lost until the same time of day on the previous day, it works out well, as the wine is presumed to have turned to vinegar from that moment.

אלא למאן דאמר מעת לעת של הנחה ⁠– האי ״מעת לעת״?! ״עד מעת לעת״ מיבעי ליה! קשיא.

But according to the one who said that this means from the time that it was set aside until the same time the day after it was set aside, this expression: From the time until the same time, is imprecise, as the presumption is that the wine turned to vinegar dating back to the day after it had been set aside. Therefore, it should have said: One is concerned going back until then from the time it was set aside to the same time the following day, i.e., 24 hours after it had been set aside. The Talmud comments: The wording of the Mishnah is difficult according to that opinion.

דברי רבי אלעזר: אמר רבי אלעזר: חלוקין עליו חביריו על רבי אלעזר ⁠– דתנן: מקוה שנמדד ונמצא חסר; כל טהרות שנעשו על גביו למפרע ⁠– בין ברשות היחיד, בין ברשות הרבים ⁠– טמאות.

The Mishnah teaches that this is the statement of the tanna R' Elazar. The amora R' Elazar says: R' Elazar’s colleagues disagree with him, as we learned in a Mishnah (Mikvaot 2:2): In the case of a mikveh that was known to have contained the requisite 40 se’a, which was then measured and found deficient in its quantity of water, all pure items that had been rendered pure in it, retroactive to when the mikveh was last measured, whether this mikveh is found in the private domain or in the public domain, are impure.