Nazir, Chapter 3, Mishnah 5. To understand this Mishnah we must first introduce a halachah that actually appears in the seventh chapter of the tractate: not all forms of corpse tumah are equal with regard to a nazir. We must distinguish between two categories:
Tumah that resets the count: a nazir who comes into contact with it must bring a sacrifice for tumah, shave his hair, and begin his nezirus again from the very start.
Tumah that does not reset the count: the days on which he was tamei do not count toward his nezirus, but the clock is not turned back. He does not bring a sacrifice for tumah; he is only required to purify himself and then continue his count.
One of the forms of tumah in the second category is being present in a cemetery without direct contact with a grave. Even so, a person who accepted nezirus upon himself while he was in a cemetery, though the clock is not reset, is taught by our Mishnah to be, in certain respects, not a full nazir.
Accepting nezirus in a cemetery:
The Mishnah deals with someone who vowed nezirus while he was in a cemetery. Even if he spent all thirty days of the nezirus there and kept all its laws, drinking no wine and not becoming tamei in any other way, "ein olin lo min haminyan" - the time he spent there does not count toward the tally of his nezirus.
True, even if he had entered a cemetery in the middle of his nezirus, that would not have obligated him in shaving and a sacrifice for tumah, nor would it have reset his count. But here he never began his nezirus at all, since he accepted it upon himself while he was already in the cemetery.
"V'eino meivi korban tumah":
The Mishnah does not mean to say that being in a cemetery is not among the forms of tumah that reset the count and obligate shaving and a sacrifice - that halachah is explained in the seventh chapter. Its intent here goes considerably further: even if he were now to become tamei with a severe tumah, such as touching a corpse, which obligates shaving and a sacrifice for tumah and resets the clock, even then he would not bring a sacrifice for tumah. The reason: he never experienced ordinary nezirus and never had days of nezirus in purity, since he began his nezirus in a cemetery.
"Yatza v'nichnas":
The Mishnah continues: he left the cemetery and purified himself, counted seven days and was sprinkled with the waters of the parah adumah, and became genuinely pure, and began to count a nezirus of purity. Afterward he returned and entered a cemetery again - "olin lo min haminyan" - the days that preceded his return, the days of purity, do count for him, and entering the cemetery does not reset the count, since, as stated, the tumah of a cemetery does not obligate shaving.
However, if he now came into contact with a corpse, a tumah that turns the clock back, "u'meivi korban tumah" - he brings a sacrifice for tumah. In this respect this case differs from the case at the beginning of the Mishnah: there, one who accepted nezirus in a cemetery would not have had the clock turned back even if he subsequently became tamei with a genuinely severe tumah, because he never experienced a nezirus of purity. Here, however, he left the cemetery and experienced days of nezirus in purity, and so when he returned to the cemetery and became tamei with a real and complete tumah, he brings a sacrifice for tumah and the clock is turned back.
The words of Rabbi Elazar:
Rabbi Elazar adds a point that applies to this case, though it holds true generally as well: "lo bo bayom" - not on that very day. If he became tamei on the very first day of his purity, meaning that he experienced only a single day of nezirus in purity, and on that very same day he became tamei with a tumah for which a nazir shaves, he is not obligated to bring a sacrifice for tumah, and even that single day is not lost from the count but is credited to him.
And from where do we know this? "Shene'emar: 'hayamim harishonim yiplu'" - as it says, "the first days shall fall." This verse is stated regarding the law of a nazir who became tamei with a genuine tumah that obligates a sacrifice, and it sets out the matter of turning the clock back: the first days fall away and are no longer counted. And the Sages expound from it: "ad shehiyu lo yamim rishonim" - until he has first days, that is, until he has a plurality of days. Only once he has more than a single day do we say that the tumah resets the count and obligates a sacrifice for tumah; but if he became tamei on the very first day, so that he has no second day, the clock is not turned back and he does not bring a sacrifice for tumah. This entire halachah applies only when he has at least two days of nezirus in purity to his credit.
In summary: in this Mishnah we learned the law of one who accepted nezirus upon himself in a cemetery: the days he spent there do not count toward his tally, and even if he later becomes tamei with a severe tumah he does not bring a sacrifice for tumah, because he never experienced days of nezirus in purity. By contrast, one who left, purified himself, counted days of purity, and then entered again - his days of purity do count toward his tally, and if he becomes tamei with a complete tumah he brings a sacrifice for tumah and the clock is turned back. And Rabbi Elazar taught that this law does not apply on the first day itself, as it says, "the first days shall fall" - until he has first days, meaning at least two days of nezirus in purity.