Rosh Hashanah 10b:10

אֵינוֹ דִּין שֶׁיּוֹם אֶחָד עוֹלֶה בִּתְחִלָּתָהּ is it not right that one day should count as a full year at the beginning of the year?

וְאֶלָּא מַאי רַבִּי אֶלְעָזָר שְׁלֹשִׁים וּשְׁלֹשִׁים בָּעֵי דִּתְנַן אֵין נוֹטְעִין וְאֵין מַבְרִיכִין וְאֵין מַרְכִּיבִין עֶרֶב שְׁבִיעִית פָּחוֹת מִשְּׁלֹשִׁים יוֹם לִפְנֵי רֹאשׁ הַשָּׁנָה וְאִם נָטַע וְהִבְרִיךְ וְהִרְכִּיב יַעֲקוֹר דִּבְרֵי רַבִּי אֶלְעָזָר רַבִּי יְהוּדָה אוֹמֵר כׇּל הַרְכָּבָה שֶׁאֵינָהּ קוֹלֶטֶת בִּשְׁלֹשָׁה יָמִים שׁוּב אֵינָהּ קוֹלֶטֶת רַבִּי יוֹסֵי וְרַבִּי שִׁמְעוֹן אוֹמְרִים שְׁתֵּי שַׁבָּתוֹת Rather, what opinion does it follow? If the baraita was not taught in accordance with the opinion of Rabbi Meir, does it follow the opinion of Rabbi Elazar, who holds that only thirty days are considered like a year? If so, then thirty days and another thirty days are required: Thirty days for the planting to take root, and another thirty days to count as a year. As we learned in a mishna: One may not plant, layer, or graft trees on the eve of the Sabbatical Year less than thirty days before Rosh HaShana, and if one planted, layered, or grafted, he must be uproot it, as the planting will take root only in the seventh year; this is the statement of Rabbi Elazar. Rabbi Yehuda says: Any grafting that does not take root within three days will never take root. Rabbi Yosei and Rabbi Shimon say: Two weeks are needed for the planting to take root.

וְאָמַר רַב נַחְמָן אָמַר רַבָּה בַּר אֲבוּהּ לְדִבְרֵי הָאוֹמֵר שְׁלֹשִׁים צָרִיךְ שְׁלֹשִׁים וּשְׁלֹשִׁים לְדִבְרֵי הָאוֹמֵר שְׁלֹשָׁה צָרִיךְ שְׁלֹשָׁה וּשְׁלֹשִׁים לְדִבְרֵי הָאוֹמֵר שְׁתֵּי שַׁבָּתוֹת צָרִיךְ שְׁתֵּי שַׁבָּתוֹת וּשְׁלֹשִׁים יוֹם וְאִי נָמֵי כְּרַבִּי יְהוּדָה סְבִירָא לֵיהּ שְׁלֹשָׁה וּשְׁלֹשִׁים בָּעֵי And on this topic Rav Naḥman said that Rabba bar Avuh said: According to the statement of the one who says thirty days, this means that it requires thirty days for the planting to take root and another thirty days to count as a year. And according to the statement of the one who says three days, this means that it requires thirty-three days. And according to the statement of the one who says two weeks, this means that it requires two weeks for the planting to take root and another thirty days to count as a year. And if the tanna of the mishna holds in accordance with the opinion of Rabbi Yehuda that only three days are needed for the planting to take root, it still requires three days for the planting to take root and thirty days to count as a year. If so, the baraita cannot be understood even in accordance with the opinion of Rabbi Elazar.

אֶלָּא לְעוֹלָם רַבִּי מֵאִיר וְכִי קָאָמַר שְׁלֹשִׁים לִקְלִיטָה Rather, it must be understood as follows: The baraita was actually taught in accordance with the opinion of Rabbi Meir, and when it says thirty days, it is referring to the time needed for the planting to take root.

אִי הָכִי שְׁלֹשִׁים וְאֶחָד בָּעֵי קָא סָבַר יוֹם שְׁלֹשִׁים עוֹלֶה לְכָאן וּלְכָאן The Gemara raises a difficulty: If so, it requires thirty-one days; thirty days for the planting to take root and one more day to count as a year. The Gemara answers: This is theoretically correct, but he holds that the thirtieth day is counted for here and for there, i.e., it counts as both the thirtieth day for taking root and as a day that is counted as a year.

אָמַר רַבִּי יוֹחָנָן וּשְׁנֵיהֶן מִקְרָא אֶחָד דָּרְשׁוּ וַיְהִי בְּאַחַת וְשֵׁשׁ מֵאוֹת שָׁנָה בָּרִאשׁוֹן בְּאֶחָד לְחֹדֶשׁ רַבִּי מֵאִיר סָבַר מִדְּאַכַּתִּי יוֹם אֶחָד הוּא דְּעָיֵיל בַּשָּׁנָה וְקָא קָרֵי לַהּ שָׁנָה שְׁמַע מִינַּהּ יוֹם אֶחָד בַּשָּׁנָה חָשׁוּב שָׁנָה Rabbi Yoḥanan said: And both of them, Rabbi Meir and Rabbi Elazar, who disagree about how much time must pass to count as a year, expounded the same verse. As the verse states: “And it came to pass in the one and six hundredth year, in the first month on the first day of the month, that the waters were dried up from off the earth” (Genesis 8:13). Rabbi Meir holds: From the fact that it was only one day into the year, as it was still the first day of the first month, and yet it is called the six hundred and first year, learn from here that one day in a year is already considered a year.

וְאִידַּךְ אִי כְּתִיב בְּשֵׁשׁ מֵאוֹת וְאַחַת שָׁנָה כִּדְקָאָמְרַתְּ הַשְׁתָּא דִּכְתִיב בְּאַחַת וְשֵׁשׁ מֵאוֹת שָׁנָה שָׁנָה אַשֵּׁשׁ מֵאוֹת קָאֵי וּמַאי אַחַת אַתְחַלְתָּא דְּאַחַת קָאָמַר And the other tanna, Rabbi Elazar, expounds the verse as follows. If it had written: In the six hundred and first year, it would be as you said. However, now that it is written: “In the one and six hundredth year," I can say that the word “year” relates to “six hundredth,” thereby teaching that it is still considered the six hundredth year. And what is meant by “one”? That it is the beginning of one year, but not that the first day counts as a year.

וְרַבִּי אֶלְעָזָר מַאי טַעְמָא דִּכְתִיב בָּרִאשׁוֹן בְּאֶחָד לַחֹדֶשׁ מִדְּאַכַּתִּי יוֹם אֶחָד הוּא דְּעָיֵיל בַּחֹדֶשׁ וְקָא קָרֵי לֵיהּ חֹדֶשׁ שְׁמַע מִינַּהּ יוֹם אֶחָד בַּחֹדֶשׁ חָשׁוּב חֹדֶשׁ וּמִדְּיוֹם אֶחָד בַּחֹדֶשׁ חָשׁוּב חֹדֶשׁ שְׁלֹשִׁים יוֹם בַּשָּׁנָה חֲשׁוּבִין שָׁנָה וְחֹדֶשׁ לִמְנוּיָיו וְשָׁנָה לִמְנוּיֶיהָ The Gemara asks: And with regard to Rabbi Elazar, what is the rationale for his opinion? From where does he learn that thirty days are counted as a year? As it is written: “In the first month on the first day of the month.” Since it was only one day into the month, and yet it is called a month, learn from here that one day in a month is already considered a month. And since one day in a month is already considered a month, likewise thirty days in a year are already considered a year, as a month is calculated according to its unit, and a year is calculated according to its unit. If one unit by which a month is calculated, i.e., a day, counts as a full month, so too, one unit by which a year is calculated, i.e., a month, counts as a full year.

מִכְּלַל דְּתַרְוַיְיהוּ סְבִירָא לְהוּ בְּנִיסָן נִבְרָא הָעוֹלָם § The Gemara comments: By inference, both of them, Rabbi Meir and Rabbi Elazar, hold that the world was created in Nisan and that the years are counted from that month, as, if the world were created in Tishrei and the count started then, the first day of the first month of the six hundred and first year would already have been six months into the year for the purpose of counting years.