When was the Book of Daniel Written?
The key evidence for the date of the book is that it contains very detailed predictions about the future (from the perspective of the sixth-century BCE character Daniel) which are uncannily correct up to a certain point in the Maccabean revolt. Then they go wildly wrong. Case in point: Daniel 10-11 review Seleucid and Ptolemaic history in detail up to the desecration of the Temple by Antiochus Epiphanes. But then 11:40-45 has him undertaking a campaign in Egypt and Palestine and dying there. There was no such campaign. And our other sources - which granted are inconsistent in details - agree that he died in Persia.
It certainly looks as though an author during the Maccabean revolt wrote up predictions after the fact (vaticinia ex eventu), but then ventured some actual predictions about the future. The latter turned out wrong, as they usually do.
This feature of correct predictions after the fact, then wrong actual predictions, is typical of historical apocalypses, including the Animal Apocalypse (1 Enoch 85-90), 4 Ezra, and The Seventh Vision of Daniel. In all three, as in the Book of Daniel, the wrong actual prediction include the coming of the final judgement, which has not happened yet!
The stories in Daniel 2-6 may have their origins in Aramaic folk traditions of the Persian Period, but the book as we have it in our Bibles (the Masoretic Text) was put together during the Maccabean Revolt, and chapters 7-12 were written then.
Main Themes of Daniel
Daniel and the End of the Ages
Daniel 1:1-7 – Why was Daniel Taken to Babylon?
Daniel 1 serves as an introduction to chapters 2-6, for which it may have been written. It sets out the themes that will be important in the stories (e.g., introduces the main characters, the training of the four friends in wisdom, Daniel's mantic talents, and the temptation to assimilation) and to some degree in the visions in 7-12.
The meaning Phil gives for the name Belteshazzar is a bit confused. The name could mean "Guard the life of the king!" or "Guard his life!" depending on how you reverse-transcribe it into Akkadian. The divine name "Bel" is not involved. Phil's translation mixes in "Bel" from the similar-looking name of King "Belshazzar" of chapter 5, which means "Bel, guard the king!"
I would be cautious about the meanings of the Babylonian names assigned to the the three friends. These, especially the first two, are more or less garbled. They certainly don't look like transcriptions of Babylonian names made in the sixth century.
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