Dan 9:1 In the first year of Darius the son of Ahasuerus, of the lineage of the Medes, who was made king over the realm of the Chaldeans
Daniel says he received this vision In the first year of Darius the son of Ahasuerus. There are a lot of different views about who this is, and it actually will play a significant role in the chronology of the prophecy that Daniel is about to receive so we should look at it in detail.
At the moment there is no historical record of such a king. There are two main options that conservative scholars take here. They say that because this king Darius is apparently the same one who began ruling directly after the fall of Babylon according to Daniel 5: 30-31