בָּעֵ֣ת הַהִ֗וא הִבְדִּ֤יל יְהוָה֙ אֶת־שֵׁ֣בֶט הַלֵּוִ֔י לָשֵׂ֖את אֶת־אֲר֣וֹן בְּרִית־יְהוָ֑ה לַעֲמֹד֩ לִפְנֵ֨י יְהוָ֤ה לְשָֽׁרְתוֹ֙ וּלְבָרֵ֣ךְ בִּשְׁמ֔וֹ עַ֖ד הַיּ֥וֹם הַזֶּֽה׃
אין יענער צײַט האָט גאָט אָפּגעשײדט דעם שבֿט לֵוִי צו טראָגן דעם אָרון פֿון גאָטס בונד, צו שטײן פֿאַר גאָט, אין צו באַדינען, און צו בענטשן אין זײַן נאָמען, ביז אױף הײַנטיקן טאָג.
Rashi
בעת ההוא הבדיל ה׳ וגו׳ AT THAT TIME THE LORD SEPARATED [THE TRIBE OF LEVI] — This is to be connected with the former narrative (that which speaks of the tablets of stone, ending at v. 5; vv. 6—8 which contain reference to Aaron’s death being interpolated for the reason given by Rashi);
Chizkuni
בעת ההיא הבדיל ה' את שבט הלוי, “at that time the Lord set the tribe of Levi apart;” this decree was issued while the people were still encamped around Mount Sinai. Moses acted upon that decree only after the Tabernacle had been erected, and it had assumed practical meaning. This would have been a number of months after he had received these instructions.
Da'at Zekenim
בעת ההיא, at that time, i.e. when the golden calf had been made, as I explained earlier.