Nedarim 63B

והתניא: ״עד ראש חדש אדר״⁠ — עד ראש חדש אדר הראשון, אם היתה שנה מעוברת ⁠— עד ראש חדש אדר השני.

And, so too, it is taught in a baraita: If one vows until Rosh Chodesh of Adar, the vow remains in effect until Rosh Chodesh of the first Adar, and if it was a leap year, it remains in effect until Rosh Chodesh of the second Adar.

מכלל דרישא לאו במעוברת עסקינן?! שמע מינה: הא דפשיטא ליה דמעברא שתא, הא דלא ידע.

The Talmud analyzes this baraita: Does the latter clause prove by inference that in the first clause, we are not dealing with a leap year? In a regular year there is only one month of Adar, so it is impossible to speak of a first or second Adar. Rather, learn from it that this latter clause is referring to a case where it is obvious to him that the year was extended, and therefore when he refers to the month of Adar he means the second Adar, whereas that first clause is referring to a case where he did not know that it was a leap year, in which case he means the first Adar.

מתני׳ רבי יהודה אומר: ״קונם יין שאיני טועם עד שיהא הפסח״⁠ — לא נתכוון זה אלא עד ליל הפסח, עד שעה שדרך בני אדם לשתות יין.

Mishnah: R' Yehuda says: In the case of one who says: Wine is konam for me, and for that reason I will not taste it until it will be Passover, it is understood that this individual intended for his vow to apply only until the night of Passover, i.e., until the time when it is customary for people to drink wine in order to fulfill the mitzva of drinking the 4 cups, but he did not intend to prevent himself from being able to fulfill this mitzva.