פּירוש אויף דניאל 1:21

And Daniel was there in greatness in Babylon until the first year of King Cyrus. According to the one who says that Hathach (Esther; 4:5, 6, 9, 10) is Daniel; and why was he called Hathach? Because he was cut off (חֲתָכוּהוּ) from his greatness; [according to his explanation] we must say that this is Cyrus I, who preceded Ahasuerus. But according to the one who says that all royal affairs were decided (נֶחְתָּכִים) by his orders, we must say that this Cyrus was Darius II, [who came] after Ahasuerus, and in the days of Darius the Mede, when they cast [Daniel] into the lions’ den, he was not demoted from his greatness, as it is said (6:29): “And this Daniel prospered during the reign of Darius and during the reign of Cyrus the Persian.” We learn that during the days of Cyrus I he was [still] in his greatness.

[Daniel 1:21] Vayehi (וַיְהִי - And [Daniel] was): The meaning is that he was in Babylon until the first year (shnat achat - שְׁנַת אַחַת) of King Cyrus. And if it had been written "until one year" (shanah achat - שָׁנָה אַחַת), it would seem that a complete year had passed. But by its saying [using the construct form] shnat achat (lit. "year of one"), the meaning is [that he was there sometime] within that year [not necessarily until its end].