Nazir 6B

אמר לך בר פדא: אימא סיפא, רבי אליעזר אומר: אינו סותר אלא שבעה. אי סלקא דעתך שלשים בעינן, נסתור כולהו!

The Talmud answers: Bar Padda could have said to you: Say the latter clause of that Mishnah: R' Eliezer says: It negates only 7 days. If it enters your mind that we require 30 days for a term of naziriteship, the nazirite should negate all of them, since he is still in the middle of his naziriteship. This proves that an unspecified naziriteship is only 29 days long.

קסבר רבי אליעזר: מקצת היום ככולו.

The Talmud responds that Rav Mattana can explain R' Eliezer’s opinion as follows: This does not prove that an unspecified term of naziriteship is less than 30 days, as R' Eliezer holds that the legal status of part of the day is like that of an entire day, and it is viewed as though the nazirite has completed his term of naziriteship at the start of the 30th day.

תנן: ״הריני נזיר מאה יום״, נטמא יום מאה ⁠— סותר את הכל. רבי אליעזר אומר: אינו סותר אלא שלשים יום. ואי סלקא דעתך סבר רבי אליעזר מקצת היום ככולו ⁠— נסתור שבעה, ואי לא סבר מקצת היום ככולו ⁠— ליסתור כולהו!

The Talmud raises a difficulty with R' Eliezer’s opinion. We learned in the Mishnah (16a): If one says: I am hereby a nazirite for 100 days, if he became impure on the one hundredth day, it negates the entire tally. He must bring offerings for having become impure, and he then begins his term of naziriteship anew. R' Eliezer says: It negates only 30 days. The Talmud comments: And if it enters your mind that R' Eliezer holds that the legal status of part of the day is like that of an entire day, the nazirite should negate 7 days because it is considered as though he became impure after completing his term of naziriteship. And if he does not hold that part of the day is like an entire day, he should negate all of the days, since he became impure during his naziriteship.