To when does "mi'Mochoras" pertain?
Rashi: It pertains to the day after Yom Kipur, when Moshe descended Har Sinai with the second Luchos. It cannot refer to an earlier date, since, before Matan Torah there were no "Chukei Elokim ve'Torosav" to teach them, 1 and from the time that the Torah was given until Yom Kipur, Moshe did not "sit down to judge the people" - seeing as he descended Har Sinai on the seventeenth of Tamuz, broke the Luchos and ascended again in the early morning of the eighteenth, 2 finally returning only on Yom Kipur.
Ramban: It pertains to the day after the events of the previous Parshah. 3
Rashbam: They always had a system of civil laws, and besides, they were taught Dinei Mamonos at Marah! Moshav Zekenim (18, citing R. Aharon) - at Marah they were commanded Stam about Dinim, but they did not get the details until Matan Torah.
Irrespective of when Yisro arrived (Refer to 18:12:2:2). And when the Mechilta explains the day after Yom Kipur (which the Torah does not mention here), it means that it must have taken place after Yom Kipur and not before - even assuming that they kept Yom Kipur before it was given at Har Sinai, since, before then, Moshe did not have a full day on which he could possibly have judged (See Ramban).
When did this Parshah take place?
Rashi: Even if Yisro came before Matan Torah, 1 this Parshah can only have been said in the second year 2 , since, after Moshe sent Yisro away (in Pasuk 27), we do not find that he returned. 3 Yet the Torah records that, when they departed from Har Sinai in the second year, Yisro was still with them, as the Torah records in Bamdibar, 10:29-31. 4
And the earlier Parshah is inserted chronologically.
Riva: Perhaps Moshe sent Yisro after Yom Kipur; the years are counted from Tishrei, so it is the second year. Or, the years are counted from Nisan, and the entire Parshah was in the first year, except for sending him, which was in the second year.
According to the Ramban in Pasuk 1, Yisro left to convert his family and returned.
Seeing as this was the day after he returned with the Luchos, and he was obligated to instruct the people about building the Mishkan, how could sit "Min ha'Boker Ad ha'Erev" (all day) in judgment? (See Sifsei Chachamim)
Rashi: The Torah is hinting that a Dayan who judges correctly, it is as if he studied Torah all day, and as if he was a partner with Hakadosh-Baruch-Hu in the Creation, about which the Torah writes "va''Yhi Erev va''Yhi Voker".
Moshe decided to separate from his wife, and Hashem agreed (Shabbos 87a). According to the opinion (Zevachim 116a) that Yisro came after Matan Torah, she came only after Hashem told Moshe (Devarim 5:28) "Amod Imadi" (do not return to your wife)!
Moshav Zekenim: He decided to separate and make a separate tent for her, even though she was in Midyan at the time.
According to the opinion (Zevachim 116a) that Yisro came after Matan Torah, were Moshe's sons not at Matan Torah?! It says about them (Divrei ha'Yamim 1:23:7) "Ravu l'Ma'alah" and it says "Heref Mimeni
Moshav Zekenim: Surely he came before Matan Torah. "Mi'Macharas" is the day after he brought the Korban that a convert is obligated to bring. 1
His question was based on Brachos 7b, which says that Hashem fulfilled his offer to make a great nation from Moshe, even though it was on condition that He destroy Yisrael. (PF)
Rashi writes that a Dayan who judges correctly, it is as if he was a partner with Hashem in the Creation. Mo'ed Katan 17a implies that judges are at a lower level than Chasidim!
Moshav Zekenim (14): Mo'ed Katan discusses judges who are not experts. 1
Moshav Zekenim: What is the great Chachmah of judging correctly? If the judge sees that one litigant is a swindler and he lies, and the other is honest and na?ve, even if the former's claim should win, he judges according to Emes.