Mishnayos Nazir Perek 3 Mishnah 1

One who said: I am hereby a nazirite, without specifying how long his term of naziriteship would last, shaves his hair on the thirty-first day after the start of his naziriteship, as an unspecified term of naziriteship lasts thirty days. And if he shaved on the thirtieth day, he has fulfilled his obligation. If he explicitly said: I am hereby a nazirite for thirty days, then, if he shaved on the thirtieth day, he has not fulfilled his obligation. Since the naziriteship would have been for thirty days even without him stating: For thirty days, this addition is understood to indicate that he will observe naziriteship for a full thirty days.

מִי שֶׁאָמַר הֲרֵינִי נָזִיר, מְגַלֵּחַ יוֹם שְׁלשִׁים וְאֶחָד. וְאִם גִּלַּח לְיוֹם שְׁלשִׁים, יָצָא. הֲרֵינִי נָזִיר שְׁלשִׁים יוֹם, אִם גִּלַּח לְיוֹם שְׁלשִׁים לֹא יָצָא: