Nedarim, Chapter 8, Mishnah 1. This mishnah deals with the time limits that apply to a vow, and with the question of how far they extend.
One who vows "this month":
One who says "this month" - "asur b'chol hachodesh" - he is forbidden for the entire month, whereas "Rosh Chodesh l'haba" - Rosh Chodesh is counted with the coming month and not with the present one; it belongs to the month that follows, and therefore he is permitted on that day.
The Gemara explains the case of a two day Rosh Chodesh: the first of those days is really the thirtieth day of the outgoing month, and it would seem to be included in the prohibition he took upon himself. Nevertheless he is permitted on that day, because in vows we follow ordinary human speech, and people call this day Rosh Chodesh - so it is considered part of the coming month, even though by the calendar reckoning it is the thirtieth day of the previous month.
"This year" and "this shavua":
"Shanah zu" - one who vows "this year" - "asur b'chol hashanah" - he is forbidden for the entire year, whereas "Rosh Hashanah l'haba" - Rosh Hashanah is counted with the coming year.
"Shavua zeh" - that is, the present shemittah cycle: "asur b'chol hashavua" - he is forbidden for the entire cycle, "u'shevi'is she'avrah" - the shemittah year is counted with the previous cycle, which is the cycle he is currently standing in, and therefore it too is included in the prohibition.
One who vows in unspecified terms:
Until now we have dealt with someone who identifies a specific time period. But if he does not identify a specific period, and instead says:
"Yom echad" - one day
"Shabbos achas" - one week
"Chodesh echad" - one month
"Shanah achas" - one year
"Shavua echad" - one shemittah cycle.
In all of these the mishnah says "asur mi'yom l'yom" - he is forbidden from day to day, meaning that the prohibition takes effect from the moment he made the vow until that same moment arrives the next day. In other words, in every case we are speaking of a full cycle: a day - twenty four hours; a week - all seven days; a month - all thirty days, and so on. The prohibition runs from the point at which he stands within the cycle until that cycle is complete.
For example: one who vows "one day" at three in the afternoon on Tuesday is forbidden until three in the afternoon the next day. And so with all the other time periods: from day to day, from month to month, and so on.
In summary: in this mishnah we learned the time boundaries of a vow. One who vows "this month" is forbidden for the entire month, and the Rosh Chodesh that follows is permitted, since it is counted with the coming month - and even the first of the two days of Rosh Chodesh is included in the leniency, because in vows we follow ordinary human speech. The same applies to "this year," where the Rosh Hashanah that follows is counted with the coming year, and to "this shavua," where the shemittah year is counted with the cycle he is standing in and is therefore forbidden. But one who vows in unspecified terms - a day, a week, a month, a year or a shemittah cycle - is forbidden from day to day, for a full cycle from the moment he made the vow until that cycle is complete.