Kepler, here's a few thoughts from Dictionaries and Commentaries. Firstly, Dan. 1:21 uses hyh "to be," which could mean "remain" or "continue." Secondly, a lot has been written about Darius the Mede. We know for a fact, Cyrus did not take up kingship immediately, for he was involved with military campaigns in Europe. The name Darius could be a title, referring to the governor Gobrayas. It would make sense that Cyrus would reward the Medes for their loyalty by appointing one of them as governor of Babylon while he was away. Thirdly, according to the Babylonian Chronicles, Daniel and friends were deported 605 BCE, the same year that Nebuchadnezzar was officially made king. This was the first of five deportations and three sieges of Jerusalem.
hyh, to be.
2. abide, remain, continue )with word of place or time) Ex 24:18 and Moses remainded in the mount forty days, etc., so 34:28 (both JE), Ju 17:4, 17:12, 1 S 6:1, 1 K 11:20, 2 K 11:3 = 2 Ch 22:12 +; also Lv 22:27, 25:28 )both H(, etc.; sq. `ad temp. remain until Dt 22:2, 1 K 11:40, 2 K 15:5, 2 Ch 5:9, 26:21, Dn 1:21 etc. BDB.
to remain, live (MHb.2 bBaba bathra 15a) Jr 13 Ru 12 Da 121 (Montgomery 139). HALOT.
21. the first year of King Cyrus. The year 538 b.c. The period of Daniel’s activity in Babylon is considerable (from 606 to 538) but not absolutely impossible. The author was probably not concerned here with the fact that this period amounts to almost seventy years, the length of time, as foretold by Jeremiah, for the Babylonian exile, that Daniel 9 will be concerned with. For a more likely reason why this chronological remark is made here, see Comment.
Comment
The first chapter of the book serves primarily as an introduction; it sets the scene for the other stories and the visions (chs. 7–12) that make up the rest of the book. The author here brings together various strands that appear as separate units in the other chapters of the book.
Therefore, the author of the first chapter summarizes Daniel’s career at the imperial court by saying that he was there from the reign of Nebuchadnezzar to the reign of Cyrus (1:21). [It should be recalled that “the first year of King Cyrus” (1:21), or 538 b.c., is not the end of Daniel’s career but simply the end of his service in the Babylonian court. Daniel experiences his final vision “in the third year of King Cyrus” (10:1), i.e. in 536 b.c. Cf. Comment: Detailed on 10:1.][1]
21. ‘And Daniel continued [when and how he was—colloquial Eng., ‘remained on’] until the first year of King Cyrus.’ The implication is that he was vouchsafed the joy of the release under Cyrus, and possibly that he like other faithful Jews returned home upon that glorious event. Such a return was understood by one form of Midrashic tradition, s. Hamburger, RE 1, 225. The contradiction with 10:l, acc. to which Dan. had a vision in Cyrus’ 3d year, in the Far Orient, is removed by the critical distinction of chapters 1–6 and 7–12 as distinct books; s. §21, a. This removes the arguments made by Marti (comm.), Jahn (comm.), Charles (comm.) against the originality of the verse. The editor of the whole book, or composer of chapters 7–12, did not observe the clash between the dates (recognized however by OG which reads ‘first year’ at 10:1). To overcome the contradiction and for the interpretation of the vb. ‘continued’ various exegetical expedients have been devised: he remained in honor, Aben Ezra (comm.) or, in the king’s gate, Hitzig (comm.); or, in prophecy, Stuart (comm.); or, in Babylon, so Jer. at 6:8, C. B. Michaelis (comm.) holding that he was then removed or exiled to Media. The Heb. vb. hyh ‘to be,’ in the sense as translated here, ‘continued,’ is fully corroborated, as noted by Hävernick (comm.) of Luther’s German Version, Moffatt,‘lived’ has the implication that Daniel died thereupon.
21. wyhy] Despite the objection of comm., this use of hyh, ‘remained, continued,’ is found elsewhere. The present phrase is exactly duplicated in Jer. 1:3; cf. Ruth 1:2 sm wyhyw, ‘they remained there.’ Cf. the translation-Greek of Test. Joseph, 11:8, ‘we were with him three months’; and with Bertholdt (comm.) the use of ἐσμέν = ζῶμεν, Acts 17:28, while Ehrlich cft. the Talmud use of hyh = ‘live,’ e.g., Baba b. 15a. The pesher frequently translate μένειν by hw’, e.g., Jn. 1:33, 40bis, 2:12.—`d] Geier notes that this prep. does not exclude the remoter future, cft. Ps. 110:1, 112:8. —kwrs] Also mss krs and so Ezr. 1:1 f.[2]
I prefer the translation of Dan. 1:21 by John J. Collins, “Daniel continued [at court] until the first year of King Cyrus.”
Comm.
“No special importance is attached
here to the first year of Cyrus, beyond the fact that it extends Daniel’s career
into the Persian era.”
“The Book of Daniel does not say what happened to Daniel after the first year of Cyrus. Josephus has him finish his career at Susa (Ant. 10.11.7 §§269-272). In later times there was a tradition that he was buried in Susa and that his grave was marked with a mausoleum.”[3]
[1] Hartman, L. F., & Di Lella, A. A. (2008). The Book of Daniel: a new translation with notes and commentary on chapters 1-9 (Vol. 23, pp. 131, 132). New Haven; London: Yale University Press.
[2] Montgomery, J. A. (1927). A critical and exegetical commentary on the book of Daniel (p. 137-139). New York: Charles Scribner’s Sons.
[3] J. J. Collins, A Commentary on the Book of Daniel – Hermeneia-series, pp. 129, 145.