Gemara

(Mishnah): If one said 'I will not taste wine today', it is forbidden only until nightfall.

If he said 'one day', 'one week'..., he is forbidden for that amount of time, from day to day.

(R. Yirmeyah): When he said 'wine is forbidden to me today', after dark he must ask a Chacham to permit his vow.

Question: What is the reason?

Answer #1 (Rav Yosef): This is a decree due to the case when he says 'one day' (lest people think that also then it is permitted at night).

Question (Abaye): If so, we should also decree when he says 'one day' due to when he says 'today'!

Answer (Rav Yosef): There is need for a decree only when he says 'today':

If when he says 'today' he is permitted at nightfall, he may come to think that the same applies when he says 'one day'.

When he says 'one day', he is forbidden for 24 hours. This will not lead him to drink on the same day when he says 'today'.

Answer #2 (to Question (h) - Ravina): R. Yirmeyah holds like R. Noson.

(Beraisa - R. Noson): Vowing is like building a private Mizbe'ach (when this is forbidden). Fulfilling the vow (not permitting it) is like offering a Korban on this Mizbe'ach. (Therefore, we fine him and require him to request permission.)

Question: If he said 'wine is forbidden to me for a day', what is the law? Is this considered like 'today', or like 'one day'?

Answer #1 (Mishnah): If he said 'wine is forbidden to me today', it is forbidden only until nightfall.

Inference: Had he said 'a day', it would be forbidden for 24 hours.

Objection (Seifa): If he said 'one day', he is forbidden for 24hours.

Inference: Had he said 'a day', he would be permitted at nightfall!

Conclusion: One of these inferences must be invalid. We cannot settle our question from the Mishnah.

Rishonim

The Rif and Rosh (8:1) bring R. Yirmeyah's law. The question about 'a day' was not settled.

Rosh: Since it was not settled, we are stringent.

Rambam (Hilchos Nedarim 10:1): If one vowed or swore 'I will not taste today', he is forbidden only until dark. If he said 'one day', he is forbidden for 24 hours from when he vowed. Therefore, if one vowed 'I will not taste today', even though he is permitted after dark, he may not eat until he asks a Chacham. This is a decree lest another time he will vow one day and eat after dark. Not everyone knows the difference between these.

Rambam (2): If one vowed 'I will not taste for a day', this is a Safek. He is forbidden for 24 hours, like one who said 'one day.' If he ate after dark he is not lashed.

Poskim

Shulchan Aruch (YD 220:1): If one said 'Wine is Konam, I will not taste it today', or 'I will not taste a day', he is forbidden for 24 hours.

R. Akiva Eiger (1): If one said 'I will not taste a day', he must begin the Neder immediately. If he eats today he transgresses Bal Te'acher (delaying a Neder). The Rashba (63a DH Konam) says that he did not transgress the Neder itself; he compensates another day. The Ran (Sof 63a) leans to say that there is no compensation, for he intended to forbid immediately. (The Rashba and Ran discuss a vow for a year. R. Akiva Eiger explains that the same applies to a day.)

R. Akiva Eiger (2): If one vowed not to eat or do something 'on Yom Ploni', it is not clear if the vow begins at night or in the morning.

Shulchan Aruch (ibid.): If he said 'I will not taste today', he is forbidden only until dark, whether he said so at the beginning or end of the day. He must ask a Chacham. This is a decree lest another time he will vow one day, and not know to distinguish between 'today' and 'one day', and drink after dark.

Shach (3): If he vowed 'one day', he must ask mid'Oraisa if he wants to eat at night, and he needs a Pesach. If he vowed 'today', he needs to ask mid'Rabanan, and does not need a Pesach.

Shulchan Aruch (ibid.): Some say that if it was a Neder of a Mitzvah, e.g. to learn or do a Mitzvah, he need not permit it.

Source (Rosh and Ran 60b DH k'Man): Ravina said that R. Yirmeyah holds like R. Noson, who says that vowing is like building a Bamah. This reason does not suffice by itself, for if so one should need to ask a Chacham after completing any vow! Rather, it is also for Rav Yosef's reason, but we would not fine for Rav Yosef's reason alone. Therefore, If one vowed to do a Mitzvah, he need not ask.

R. Akiva Eiger (3): According to this, if Shimon forbade Levi, we should not fine Levi, for he did nothing wrong. However, the Rosh and Ran did not mention this. This suggests that it is not true. This requires investigation.

Lechem Mishneh (Nedarim 10:1): The Rambam did not distinguish between vows of Mitzvah or Reshus. He holds, unlike the Rosh and Ran, that Ravina disagrees with Rav Yosef's reason. Ravina decrees only about a vow of a day because perhaps he meant 24 hours. He rules like Rav Yosef, for his reason forbids all such vows for 'today', and the Mishnah did not distinguish.

Rema: If one accepted to fast, he may eat at night immediately. A Chacham need not permit it.

Source (Ran 60b DH u'Mah): We need not decree about fasts, for everyone knows that they are only until dark. They are unlike other vows, for which Chachamim fixed times. Therefore, one will not come to be lenient about a fast for one day. The Rashba says that we decree lest one equate 'one day' to 'today', for in both cases he was permitted to eat part of this day. He will not equate 'one day' to a fast, in which the entire day was forbidden. This is wrong. We discuss even one who vowed before dawn, and was forbidden the entire day! Therefore, even If one vowed to fast 'one day in the week', he may eat after dark, for he intended like fasts of Chachamim.

Defense (of Rashba - Beis Yosef DH ul'Aniyas): There is a need to decree when he vowed in the middle of the day. Chachamim made a general decree and did not distinguish at what time of day he vowed.