Gemara

69b - Question #1 (Rabah): If he (a father or husband) said 'your vow is affirmed today', what is the law?

Is it as if he said 'it is annulled tomorrow'?

Or perhaps it is not, for he did not say so explicitly?

Question #2: If you will say that it is not annulled, if he said 'it is annulled tomorrow', what is the law?

Do we say that he cannot annul tomorrow, for he affirmed today?

Or, since he did not say that he affirms today, he really means that he annuls today?

Question #3: If you will say that since he affirmed it today, it is as if the vow persists tomorrow, what if he said 'it is affirmed for an hour'?

Is it as if he said 'it is annulled after an hour'?

Or, it is not, for he did not say so explicitly?

Question #4: If it is not as if he said 'it is annulled after an hour', what if he explicitly said so (after affirming for an hour)?

Once he affirmed it, it is affirmed;

Or, since the whole day he can affirm or annul, it is annulled?

Answer (Mishnah): If a woman said 'I am a Nezirah' and her husband said 'also I', he cannot annul her Nezirus;

Question: Why don't we say that he affirmed her Nezirus only for a short time, for the sake of Hatfasah (to make himself a Nazir like her), and later he can annul it if he wants?

Answer #1: This teaches that Kiyum (even for a short time) is Kiyum.

Rejection (and Answer #2): Saying 'also I' shows that he wants to permanently affirm her Nezirus.

Rishonim

Rif and Rosh (10:4): Our question was not resolved, therefore we are stringent.

Rambam (Hilchos Nedarim 13:22): If a man told his daughter or wife 'your vow is affirmed today', it is permanently affirmed. If he said 'it is annulled tomorrow', it is not annulled, for he affirmed today, and he cannot annul tomorrow. If he said 'it is affirmed for an hour' and he did not annul the entire day, it is affirmed. We do not consider it as if he annulled it after an hour, for he did not say so explicitly. If he said 'it is affirmed for an hour' and after an hour he annulled it, it is a Safek. Therefore, she may not transgress her vow, but if she did she is not lashed.

Kesef Mishneh: The Rambam holds that whenever it says 'if you will say...', this is the Halachah. Therefore, only the last case is a Safek.

Lechem Mishneh: The Gemara connotes that that he said now that it should be annulled after an hour. The Rambam holds that the same Safek applies if he annulled after the hour.

Gra (YD 234:103): The Shulchan Aruch (which cites the Rambam) holds that the Gemara discusses this case, i.e. he said 'it is affirmed for an hour' and after an hour he annulled it.

Poskim

Shulchan Aruch (YD 234:45): If he told her 'your vow is affirmed today', 'it is permanently affirmed.

Shulchan Aruch (46): If he said 'it is annulled tomorrow', it is not annulled.

Rema: Some say that this is Kiyum.

Question (Taz 40): The first opinion agrees! We cannot say that the first opinion holds that one can annul afterwards, for the Gemara said 'if you will say that since he affirmed it today, it is as if the vow persists tomorrow.' The first opinion is the Rambam (who rules like every 'if you will say.')

Answer (Nekudas ha'Kesef): The Rema deduced that the Rambam did not say that 'your vow is annulled tomorrow' is Kiyum, like he said about 'your vow is affirmed today'. The Rambam learned from the Gemara, which asked whether or not he can annul tomorrow. Why didn't it ask whether or not he can annul immediately after saying this? Later, the Gemara says 'it is as if the vow persists tomorrow.' This connotes that the question is about tomorrow. It is clear that he can annul immediately, for what he said is not Kiyum. The question is whether he affirmed today (by not properly annulling), or if he means that tomorrow it will be annulled from today (if he will annul tomorrow).

Shulchan Aruch (46): If he said 'it is annulled tomorrow', it is not annulled.

Shulchan Aruch (47): If he said 'it is affirmed for an hour' and he did not annul the entire day, it is affirmed.

Question (Bach DH v'Ika): Why did the Tur omit the third case, in which he said only 'it is affirmed for an hour'? It seems that there is a omission in the text of the Tur. This case should be included with the last, 'it is affirmed for an hour, and after an hour it is annulled.' Both of these are Sefekos.

Rebuttal (Taz 39): The Tur did not need to bring this case, for in the last case even when he adds 'and after an hour it is annulled', it does not help. The Rambam brought it because he holds that the first three cases were all settled (that it is affirmed), unlike the Tur, who is stringent due to Safek in all four cases.

Shulchan Aruch (48): If he said 'it is affirmed for an hour' and after an hour he annulled it, it is a Safek.

Bach (DH Omar): We are stringent about Safek Isur. For us (we do not lash nowadays), it is as if also this case was Vadai forbidden.

Rebuttal (Korban Nesan'el 10:10): There is a difference even nowadays! If one was Mekadesh her on condition that she has no Nedarim, if the Neder was Vadai affirmed, she is not Mekudeshes. If the Neder was Safek annulled, she is Safek Mekudeshes.