Nazir, Chapter 1, Mishnah 3. This mishnah deals with the length of the nezirus period - how long a person's nezirus lasts, both when he specified its length and when he did not.
"Stam nezirus sheloshim yom":
The mishnah opens by stating: "Stam nezirus sheloshim yom" - an unspecified nezirus is thirty days. An "unspecified nezirus" is one in which the person does not spell out how long he wants it to last: he says "I am hereby a nazir" without indicating for how long. In such a case his nezirus is thirty days.
The Rambam explains that this law is a halachah received by tradition, a halachah given to Moshe at Sinai, except that the Sages found support for it in a verse: it says "kadosh yihyeh" (he shall be holy), and the word "yihyeh" has the gematria of thirty - from here we learn that an unspecified nezirus is thirty days.
"Achas gedolah" and "achas ketanah":
The mishnah continues: if he said "Hareini nazir achas gedolah" - "I am hereby a nazir, one long one" - he has created a single nezirus, and it is a long one; or if he said "Hareini nazir achas ketanah" - "I am hereby a nazir, one short one" - a single nezirus, and it is a short one; "va'afilu mikan ve'ad sof ha'olam" - and even if, when accepting the nezirus, he added that it is so long that it stretches from here to the end of the world. With any of these expressions he is a nazir for thirty days only.
The reason: his saying "a long one," and even his saying "until the end of the world," does not fix any period of time. It merely expresses his attitude toward the nezirus - that it seems to him weighty and great. But as far as its actual length is concerned, it is no more than thirty days, and this is an unspecified nezirus.
"Hareini nazir ve'yom echad" - two neziruyos:
In contrast, in three cases he is a nazir twice over:
"Hareini nazir ve'yom echad" - "I am hereby a nazir and one day": he accepted upon himself a nezirus and one additional day.
"Hareini nazir ve'sha'ah achas" - "I am hereby a nazir and one hour": he accepted upon himself a nezirus and one additional hour.
"Hareini nazir achas u'mechtzah" - "I am hereby a nazir, one and a half": he accepted upon himself one nezirus and half a nezirus.
In each of these cases he is a nazir twice - two neziruyos, two periods of thirty days. The reason is that the first statement, "I am hereby a nazir," is a declaration of a complete nezirus of thirty days; whereas the addition - one day, one hour, or half a nezirus - has no independent existence, since there is no nezirus of a single day, no nezirus of a single hour, and no such thing as half a nezirus. The addition therefore means a second nezirus, and so he is a nazir twice.
"Hareini nazir sheloshim yom ve'sha'ah achas":
In contrast to this the mishnah says: one who says "Hareini nazir sheloshim yom ve'sha'ah achas" - "I am hereby a nazir for thirty days and one hour" - is a nazir for thirty-one days. Here he does not say "I am hereby a nazir" without qualification; rather, he explicitly fixes a nezirus of thirty days and adds an hour to it. Therefore he is not a nazir for two neziruyos but for thirty-one days, "she'ein nezirus le'sha'os" - because there is no nezirus for hours, that is, a nezirus of an hour alone has no existence.
The difference between the earlier cases and this one: there he made a complete, separate nezirus and only afterward added an hour or a day, and that addition creates a second nezirus. But here, once he said that he is a nazir for thirty days plus an hour, that hour joins the thirty-day period itself and does not create a second nezirus. And since there is no nezirus for an hour, but there is a nezirus of thirty-one days or of thirty-two days - for nezirus is counted in days and not in hours - the hour is not treated as a mere hour, but is accepted as a full day added to the thirty-day period.
In summary: in this mishnah we learned that an unspecified nezirus is thirty days, a halachah given to Moshe at Sinai that was supported by the gematria of "yihyeh"; that the expressions "one long one," "one short one," and "from here to the end of the world" add nothing to the thirty days but express an attitude only; that one who adds to an unspecified nezirus a day, an hour, or a half is a nazir twice, since the addition has no existence on its own; whereas one who fixes thirty days and an hour is a nazir for thirty-one days, because there is no nezirus for hours and the hour counts as a full day.