What are the most significant arguments for an early date of authorship of Daniel (6th century BC)? What are the primary arguments for a late date (2nd century BC)?
Arguments for a late (2nd century BC) dating:
Arguments for an early (6th century BC) dating:
Argument from Tradition
To give a flavor of the debate over loan words, here is an excerpt from Wikipedia:
Further argument for a late date
Not only does Daniel seem able to prophesy events close to the time of 167 BCE accurately, although not the relevant events that occurred shortly after this time, but its narrative around the chronology of the Exile seems flawed. Chapter 8 is in the time of Babylonian rule, then Daniel 9:1 is the first year of Darius, son of Xerxes, who was made king over the Chaldeans (Babylonians). "Made king over the Chaldeans" implies conquest, but it was Cyrus who conquered the Chaldeans. Allowing that "son of Xerxes" could mean descendant of, the first Darius who was a descendant of Xerxes I was Darius II, who lived 423-404/5 BCE, far too late for Daniel.
My conclusion is that i) the book was not written after about 167 BCE, else it would certainly have mentioned those events, nor was it predictive, or once again it would have mentioned those events; ii) it was not written during the Exile or early Persian period, or it would have been at least as accurate regarding events in this period as it was regarding events leading up to 167 BCE.
More on the linguistics for an early date. It is more proper to say that Hebrew had changed drastically by the 2nd century BC. Lingual shifts had happened but it was still a living language. The most obvious being the word order changed from verb-subject-object to subject-verb-object. It is called Mishnaic Hebrew and the rabbinic writings are full of it. Evidence for Hebrew as a living language throughout the time of Jesus can be found in Understanding the Difficult Words of Jesus. Also, coins have been found from the Hasmonean period (165 BC-37 BC) with Hebrew writing on them. During the time of the Hasmoneans, a Hebrew renaissance was in full swing and a new book written in Aramaic would not easily have been accepted.
The Hebrew in Daniel is perfect for Biblical Hebrew but does not match what was used in Mishnaic Hebrew. Likewise, Aramaic had changed since the time of Imperial Aramaic. The Aramaic in Daniel is perfect for Imperial.
One last indication of the antiquity of Daniel is the style of the apocalyptic sections. Apocalyptic writings changed heavily over the life of the genre, and we have examples of them from throughout the Intertestamental period. Daniel's apoc sections don't have the pieces we would expect from a late dating. For example, Daniel lacks an angelic guide which was so popular in the later apocalyptic writings. In fact, Zechariah has an angelic interpreter.