Nedarim 88 - The husband did not know
If the husband did not know in principle that he can annul his wife's vows - even though he understood the concept of vows - then his failure to confirm or revoke is meaningless. Therefore, once he finds out that he has this capability - then he has a day to revoke the vow or confirm it.
However, if he knew about revocation, just did not know that a specific vow could be revoked by him - then it's his problem. Even if he revokes this vow, just to be on the safe side, this revocation does not work, because he lacks complete knowledge. And later it would be too late, because the first day passed. Why is this case different? - He should have gone to a Sage to clarify, on that very day. This is the view of Rabbi Meir.
The Sages, however, disagree: since he knew about vow revocation, but just did not know that this particular vow can be revoked by him - and he revoked it just in case - it works. The Sages consider partial knowledge the same as complete knowledge. Afterwards he cannot revoke it any longer, since the first day of hearing has passed.
Art: Portrait of a Husband and Wife by John Parker