cellio | daf bit: Nedarim 60

The mishna teaches: if one vows to abstain from wine "today" he is forbidden until dark; "this Shabbat" means he is forbidden through the end of Shabbat of this week; "this month" means he is forbidden through the month but Rosh Chodesh (the first of the next month) belongs to the next month, not this one; "this seven-year cycle" means he is forbidden through the sh'mita year (the year that we let fields lie fallow). So, Shabbat and the sh'mita year bind to what comes before, but a new day or a new month binds to what comes after.

In the g'mara, R' Yirmiyahu says: at nightfall he must be released from his vow (for "today") by a sage. Why? Because if he had instead said "one day" instead of "today", he would be forbidden for a full 24 hours, and in that case we wouldn't want him thinking that "day" vows expire at nightfall, lest he violate his vow unintentionally. (60a)

I'm surprised that the list includes day, week, month, and seven-year cycle, but not year. One could reasonably argue that it's obvious because a new year begins with a new month, so you'd follow the Rosh Chodesh rule, but I don't see that discussion here. (I haven't read ahead, though, so maybe it's coming.)

daf bit: Nedarim 60

Way off on a tangent, here