V1 In the first year of Belshazzar Daniel had a Dream and visions of his head upon the bed.then he wrote the dream, and told the sum of the matters.
The exact date for the first year of King Belshazzar's reign is somewhat speculative. Historians place the first year of his reign about 541 BC. Daniel would be well into his 80's at this time. This dream took place 62 years after Daniel first interpreted Nebuchadnezzar's statue dream in 603 BC. Daniel's dream took place before the events which are recorded in chapter 5 and at least 2 years before Daniel's second dream which is outlined in Chapter 8.
The verse indicates he is about to relay the details which he experienced while sleeping. He was either dreaming or what he saw was pictured in his mind rather than with his eyes. The reader must realize that whether what he saw was a vision or a dream both come from God, and both are equally creditable. Then he wrote the dream and told the sum of the matters. He immediately wrote down the details and told the sum of the matters. I believe he did this so he would not forget what he saw. If a person wanted to fabricate a prophetical prediction, writing it down is not the best choice to preserve credibility in the event the prediction is not exactly true. Charlatans speak in riddles, steer clear from specifics, and change details as often as necessary. Daniels goal was to preserve the details of the dream/vision which he saw; it was God’s responsibility to fulfill the facts that surround this prophetic occurrence.