There we learned: they grant a virgin twelve months from when the husband has claimed her, in order to provide for herself. From where are these words derived? For Scripture says, "Let the maiden remain with us days, or ten" (Genesis 24:55). What is "days"? If you say two days - does a person speak this way, saying to him "two days," and he says to them "no," and then they say to him "ten days"? Rather, what is "days"? A year, as it is written, "Days shall be the time of his redemption" (Leviticus 25:29). But say, what is "days"? A month, as it is written, "until a month of days" (Numbers 11:20). We derive plain "days" from plain "days," and we do not derive plain "days" from "days" concerning which the term "months" is stated [the gezeira shava links the two unqualified uses of "days"].
Twelve Months Granted a Betrothed Virgin
Yalkut Shimoni on Torah 109:8
תַּמָּן תָּנִינָן נוֹתְנִין לִבְתוּלָה שְׁנֵים עָשָׂר חֹדֶשׁ, מִשֶׁתָּבְעָהּ הַבַּעַל כְּדֵי לְפַרְנֵס אֶת עַצְמָהּ מִנָּא הֲנֵי מִילֵי, דְּאָמַר קְרָא תֵּשֵׁב הַנַּעֲרָה אִתָּנוּ יָמִים אוֹ עָשׂוֹר, מַאי יָמִים, אִילֵימָא תְּרֵי יוֹמֵי, מִשְׁתָּעֵי אִינִשׁ הָכֵי אָמְרוּ לֵיהּ, תְּרֵי יוֹמֵי וְאָמַר לְהוּ לָאו, וְהָדַר אָמְרוּ לֵיהּ עֲשָׂרָה יוֹמֵי אֶלָּא מַאי יָמִים, שָׁנָה דִּכְתִיב, (ויקרא כה, כט) "יָמִים תִּהְיֶה גְּאוּלָתוֹ". וְאֵימָא מַאי יָמִים, חֹדֶשׁ דִּכְתִיב (במדבר יא, כ) "עַד חֹדֶשׁ יָמִים", דָּנִין יָמִים סְתָם מִיָּמִים סְתָם, וְאֵין דָּנִין יָמִים סְתָם מִיָּמִים שֶׁנֶּאֱמַר בָּהֶם חֳדָשִׁים.