(Mishnah): Hafaras Nedarim is the entire day. This can be a leniency or a stringency:

How is this? If she vowed on Shabbos night, he can annul on Shabbos night, and Shabbos day until dark. If she vowed shortly before nightfall, he can only annul until nightfall.

(Beraisa - R. Yosi bei Rebbi Yehudah and R. Lazar bei R. Shimon): Hafaras Nedarim is me'Es la'Es (24 hours).

What is the source for Rabanan (our Mishnah)? "B'Yom Shom'o."

What is the source for R. Yosi bei Rebbi Yehudah [and R. Elazar bei R. Shimon]? "Mi'Yom El Yom."

Question: How does R. Yosi bei Rebbi Yehudah establish Chachamim's source "mi'Yom El Yom"?

Answer: The case is, she vowed at the beginning of the night.

Question: How do Rabanan establish the source of R. Yosi bei Rebbi Yehudah "b'Yom Shom'o"?

Answer: The case is, she vowed at the beginning of Shabbos night, and he became mute, and returned to speak (on Shabbos);

According to R. Yosi bei Rebbi Yehudah, we give to him 24 hours [to annul from when his speech returned]. According to Rabanan, he has only [the rest of] that day.

If he became mute [and his speech returned on another day,] according to R. Yosi bei Rebbi Yehudah, we join for him 24 hours (from when he heard until he became mute, and from when he returned to speak). According to Rabanan, he can always annul until [the end of the day on which] he returned to speak. (Since he was mute at the end of the day he heard, that day does not count. It is as if he heard only today.)

[If she vowed] a short time before Shki'ah, he cannot annul [after Shki'ah].

[If he said] 'it is annulled for you at Minchah (in the afternoon)', it is permanently annulled (do not say that he affirmed it until then, and it is permanently affirmed). 'It is affirmed for you at Minchah', it is permanently affirmed (do not say that he annulled it until then, and it is permanently annulled).

'It is annulled for you until Minchah', it is as if he said 'it is annulled for you from Minchah and onwards.'