Why the Minimum Nazirship Length?

Question

In Nazir 2:10, the Mishna states:

:אֵין תִּגְלַחַת פָּחוֹת מִשְּׁלשִׁים יוֹם

There can be no shaving for a period of less than thirty days.

Meaning, if you were in the middle of one nazirship when you started a second one, and you had less than thirty days left of the first nazirship, you would still need to finish the first nazirship thirty days after the end of the second one. This is because you cannot end two nazirships within thirty days of each other.

What is the source/reason for the thirty-day minimum between ending nazirships? A nazirship must be at least thirty days, but surely the days of the first nazirship before the second one would count for that!

Answer 1

במדבר ו ה

כָּל־יְמֵי֙ נֶ֣דֶר נִזְר֔וֹ תַּ֖עַר לֹא־יַעֲבֹ֣ר עַל־רֹאשׁ֑וֹ עַד־מְלֹ֨את הַיָּמִ֜ם אֲשֶׁר־יַזִּ֤יר לַיהוָה֙ קָדֹ֣שׁ יִהְיֶ֔ה גַּדֵּ֥ל פֶּ֖רַע שְׂעַ֥ר רֹאשֽׁוֹ׃

Bamidbar p6 5

Throughout the term of his vow as nazirite, no razor shall touch his head; it shall remain consecrated until the completion of his term as nazirite of the LORD, the hair of his head being left to grow untrimmed.

ספרי על במדבר ו ה

גדל פרע שער ראשו . ... כמה הוא גידול פרע, אין פחות משלשים יום. ...

Sefri Bamidbar p6 5

his head being left to grow untrimmed - ... how much is it, to be considered "untrimmed"? no less than 30 days...

A person must achieve "untrimmed" hair every Nezirut, and one can't reuse his/her hair for another Nezirut because:

במדבר ו יח

וְגִלַּ֣ח הַנָּזִ֗יר פֶּ֛תַח אֹ֥הֶל מוֹעֵ֖ד אֶת־רֹ֣אשׁ נִזְר֑וֹ וְלָקַ֗ח אֶת־שְׂעַר֙ רֹ֣אשׁ נִזְר֔וֹ וְנָתַן֙ עַל־הָאֵ֔שׁ אֲשֶׁר־תַּ֖חַת זֶ֥בַח הַשְּׁלָמִֽים׃

Bamidbar p6 18

The nazirite shall then shave his consecrated hair, at the entrance of the Tent of Meeting, and take the locks of his consecrated hair and put them on the fire that is under the sacrifice of well-being.