Why doesn’t רבי שמעון בר נחמי accept רב שמן בר אבא explanation about בזמניהם?

Question

On Megilla 2a there's a disagreement between רבי שמעון בר נחמי & רב שמן בר אבא about where in the Torah is it hinted that we read the Megilla on the 11th, 12th, or 13th of Adar. I have thought of a question on רבי שמעון בר נחמי who holds we learn it out from "כימים":

The גמרא:

הֵיכָא רְמִיזָא? אָמַר רַב שֶׁמֶן בַּר אַבָּא אָמַר רַבִּי יוֹחָנָן, אָמַר קְרָא: ״לְקַיֵּים אֵת יְמֵי הַפּוּרִים הָאֵלֶּה בִּזְמַנֵּיהֶם״, זְמַנִּים הַרְבֵּה תִּקְּנוּ לָהֶם. הַאי מִיבַּעְיָא לֵיהּ לְגוּפֵיהּ! אִם כֵּן לֵימָא קְרָא ״זְמַן״, מַאי ״זְמַנֵּיהֶם״ — זְמַנִּים טוּבָא. וְאַכַּתִּי מִיבְּעֵי לֵיהּ: זְמַנּוֹ שֶׁל זֶה לֹא כִּזְמַנּוֹ שֶׁל זֶה! אִם כֵּן, לֵימָא קְרָא ״זְמַנָּם״, מַאי ״זְמַנֵּיהֶם״ — שָׁמְעַתְּ מִינַּהּ כּוּלְּהוּ. אֵימָא זְמַנִּים טוּבָא! ״זְמַנֵּיהֶם״ דּוּמְיָא דִּ״זְמַנָּם״: מָה ״זְמַנָּם״ תְּרֵי — אַף ״זְמַנֵּיהֶם״ תְּרֵי. וְאֵימָא תְּרֵיסַר וּתְלֵיסַר? כִּדְאָמַר רַב שְׁמוּאֵל בַּר יִצְחָק: שְׁלֹשָׁה עָשָׂר זְמַן קְהִילָּה לַכֹּל הִיא, וְלָא צְרִיךְ לְרַבּוֹיֵי, הָכָא נָמֵי: שְׁלֹשָׁה עָשָׂר זְמַן קְהִילָּה לַכֹּל הִיא, וְלָא צְרִיךְ לְרַבּוֹיֵי. וְאֵימָא שִׁיתְּסַר וְשִׁיבְסַר! ״וְלֹא יַעֲבוֹר״ כְּתִיב. וְרַבִּי שְׁמוּאֵל בַּר נַחְמָנִי אָמַר, אָמַר קְרָא: ״כַּיָּמִים אֲשֶׁר נָחוּ בָהֶם הַיְּהוּדִים״. ״יָמִים״, ״כַּיָּמִים״ — לְרַבּוֹת אַחַד עָשָׂר וּשְׁנֵים עָשָׂר. וְאֵימָא תְּרֵיסַר וּתְלֵיסַר! אָמַר רַב שְׁמוּאֵל בַּר יִצְחָק: שְׁלֹשָׁה עָשָׂר זְמַן קְהִילָּה לַכֹּל הִיא, וְלָא צְרִיךְ לְרַבּוֹיֵי. וְאֵימָא שִׁיתְּסַר וְשִׁיבְסַר! ״וְלֹא יַעֲבוֹר״ כְּתִיב. רַבִּי שְׁמוּאֵל בַּר נַחְמָנִי מַאי טַעְמָא לָא אָמַר מִ״בִּזְמַנֵּיהֶם״? ״זְמַן״ ״זְמַנָּם״ ״זְמַנֵּיהֶם״ לָא מַשְׁמַע לֵיהּ. וְרַב שֶׁמֶן בַּר אַבָּא מַאי טַעְמָא לָא אָמַר מִ״כַּיָּמִים״? אָמַר לָךְ: הָהוּא לְדוֹרוֹת הוּא דִּכְתִיב.*

Where is the allusion to this in the Bible? The Megilla itself, which was approved by the members of the Great Assembly, mentions only the fourteenth and fifteenth of Adar. Rav Shemen bar Abba said that Rabbi Yoḥanan said: It is alluded to when the verse states: “To confirm these days of Purim in their times” (Esther 9:31). The phrase “in their times” indicates that they enacted many times for them and not only two days. The Gemara objects: This verse is necessary for its own purpose, to teach that the days of Purim must be observed at the proper times. The Gemara responds: If so, let the verse say: To confirm these days of Purim in its time. What is the significance of the term “their times,” in the plural? It indicates that many times were established for the reading of the Megilla. The Gemara objects: But still, the plural term is necessary to indicate that the time of this walled city is not the same as the time of that unwalled town, i.e., Purim is celebrated on different days in different places. The Gemara answers: If so, let the verse say: Their time, indicating that each place celebrates Purim on its respective day. What is the significance of the compound plural “their times”? Learn from this term that although the verse (Esther 9:21) specifies only two days, the Megilla may, at times, be read on all of the days enumerated in the Mishna. The Gemara asks: If so, say that the plural term indicates many times, and the Megilla may be read even earlier than the eleventh of Adar. The Gemara rejects this argument: The compound plural “their times,” should be understood as similar to the simple plural term, their time. Just as the term their time can be understood to refer to two days, indicating that each location reads the Megilla in its respective time on the fourteenth or the fifteenth of Adar, so too, “their times” should be understood as referring to only two additional days when the Megilla may be read. The Gemara asks: Say that these two added days are the twelfth and the thirteenth of Adar. How is it derived that the Megilla may be read on the eleventh as well? The Gemara answers: It is as Rav Shmuel bar Yitzḥak said in a different context: The thirteenth of Adar is a time of assembly for all, as it was on that day that the Jews assembled to fight their enemies, and the main miracle was performed on that day. Consequently, there is no need for a special derivation to include it as a day that is fit for reading the Megilla. Here too, since the thirteenth of Adar is a time of assembly for all, there is no need for a special derivation to include it among the days when the Megilla may be read. The Gemara objects: And say that the two additional days are the sixteenth and the seventeenth of Adar. The Gemara responds: It is written: “And it shall not pass” (Esther 9:27), indicating that the celebration of Purim is not delayed until a later date. Having cited and discussed the opinion of Rav Shemen bar Abba, the Gemara cites another answer to the question of where the verses allude to the permissibility of reading the Megilla on the days enumerated in the Mishna. And Rabbi Shmuel bar Naḥmani said: These dates are alluded to when the verse states: “As the days on which the Jews rested from their enemies” (Esther 9:22). The term “days” is referring to the two days that are explicitly mentioned in the previous verse, i.e., the fourteenth and the fifteenth. The term “as the days” comes to include two additional days, i.e., the eleventh and twelfth of Adar. The Gemara asks: And say that the two additional days are the twelfth and thirteenth of Adar. How is it derived that the Megilla may be read on the eleventh as well? In answer to this question, Rav Shmuel bar Yitzḥak said: The thirteenth of Adar is a time of assembly for all, and there is no need for a special derivation to include it as a day fit for reading. The Gemara objects: Say that these additional days are the sixteenth and seventeenth of Adar. This suggestion is rejected: It is written: “And it shall not pass.” Since two derivations were offered for the same matter, the Gemara asks: What is the reason that Rabbi Shmuel bar Naḥmani did not state that the days enumerated in the Mishna are fit for reading the Megilla based upon the term “in their times,” in accordance with the opinion of Rav Shemen bar Abba? The Gemara answers: He does not learn anything from the distinction between the terms time, their time, and their times. Therefore, the verse indicates only that there are two days when the Megilla may be read. The Gemara asks: And what is the reason that Rav Shemen bar Abba did not state that the days enumerated in the Mishna are fit for reading the Megilla based upon the term “as the days,” in accordance with the opinion of Rabbi Shmuel bar Naḥmani? The Gemara answers: He could have said to you: That verse is written as a reference to future generations, and it indicates that just as the Jews rested on these days at that time, they shall rest and celebrate on these days for all generations. § With regard to the Mishna's ruling that the Megilla may be read on the day of assembly,

רבי שמעון בר נחמי learns out from the word "כימים", that we read the מגילה on the 11th and 12th, unlike רב שמן בר אבא, who learns it out from the word "בזמניהם".
רב שמן בר אבא doesn't hold like רב שמעון בר נחמי because he already uses the word "כימים" to teach that we should read on 14th and 15th through every generation. But according to רבי שמעון בר נחמי, why doesn't he hold like רב שמן בר אבא?

Answer 1

Welcome. I second @Isaac Moses's comment. That said, I will give an answer a shot:

1. The Gemara itself askes this questions and and answers that Rav Shimon bar Nachmani he does not think there is a significant difference to be learned between zmanam and zmaneihem. Perhaps because he thinks if there are multiple days then "zmanam" would be inappropriate because it would imply that multiple days share one zman (compare the first Tosafot on Pesachim 99b that finds similar language problematic).
2. This is really the same issue. He doesn't think there are extra letters. (Or perhaps, as @Shalom mentioned in comments above, he doesn't learn from little extra details like this).
3. The gemara asks on 2b how do we know walled and unwalled cities read on separate days. The stam answers that it's from the phrase zmaneihem, which teaches there are multiple times. So presumably Rav Shimon bar Nachmani would agree with that.

The way I understand the "zman - zmanam - zmaneihem" drash is as follows: if there was just one day, the pasuk would just say zmano. Everyone agrees that the plural language means there are at least two different days. But Rav Shemen bar Ada makes an additional drash that there are two more days because he thinks there is a "double plural," whereas Rav Shimon bar Nachmani thinks zmaneihem is a regular, grammatically correct plural (or doesn't care about that fine distinction).

Now, as for question 3 it's less clear. I think the simplest answer is that there are lots of other cues that Purim was accepted in perpetuity. For example, the passage where we find "zmaneihem" reads:

29 So Queen Esther, daughter of Abihail, along with Mordecai the Jew, wrote with full authority to confirm this second letter concerning Purim. 30 And Mordecai sent letters to all the Jews in the 127 provinces of Xerxes’ kingdom—words of goodwill and assurance— 31 to establish these days of Purim at their designated times, as Mordecai the Jew and Queen Esther had decreed for them, and as they had established for themselves and their descendants in regard to their times of fasting and lamentation.

Esther 9:29-31. So he may just feel that "kayamim" is not needed to teach Purim for subsequent generations because we learn it elsewhere. If anything, I wonder why Rav Shemen bar Abba thinks we need kayamim to teach Purim for future generations.