Daniel purports to be the account of a high ranking Jewish official in the court of Nebuchadnezzar. The first half is largely heroic stories where the protagonists are protected from certain death by adhering strictly to Jewish law (e.g. not bowing to false gods). The second half is a series of increasingly-detailed prophecies about the coming four or five hundred years of history.
In reality the book is more likely a pious fraud written during the Maccabean revolt in the 2nd century BC (circa 165 B.C). Besides the obvious fact that it is natural to be skeptical of claims of prophecy, the following reasons are usually given to support the claim that this is not authentic prophecy.
- The history of Babylonia reported in Daniel doesn't match up with extra-biblical cources (e.g., it lists the wrong king of Judah when Nebuchadnezzar brings the Jews into exile).
- The predictions between the 6th and 2nd centuries BC don't match well with the modern understanding of the history of that time period. (The author of Daniel believed that the Medes were more powerful than Persians, whereas they had been conquered by the Persians).
- The predictions about the early part of Maccabean revolt against Antiochus IV are suddenly very accurate (battles, forced Hellenization of Jews, etc)
- The later predictions about the Maccabean revolt are suddenly not accurate again. (E.g. the imminent end of the world didn't occur, Antiochus didn't die where predicted).
Daniel is a favorite of evangelical end times theorists, since its often vivid imagery can be matched, Rorschach-like, to just about any current event.