Another interpretation: when he would see the angels praising Him with "Holy," he knew it was day; and at the hour when he would see them praising with "Blessed," he knew it was night.
How Moses Knew Day from Night by the Angels' Praise
Yalkut Shimoni on Torah 406:2
דָּבָר אַחֵר. כְּשֶׁהָיָה רוֹאֶה לַמַּלְאָכִים מְקַלְּסִין אוֹתוֹ בְּקָדוֹשׁ, הָיָה יוֹדֵעַ שֶׁהוּא יוֹם, וּבְשָׁעָה שֶׁהָיָה רוֹאֶה אוֹתָן מְקַלְּסִין בְּבָרוּךְ, הָיָהּ יוֹדֵעַ שֶׁהוּא לָיְלָה.