Daniel saw:

I saw in the night visions, and behold, with the clouds of heaven there came one like a son of man, and he came to the Ancient of Days and was presented before him. -- Daniel 7:13 (ESV)

Then Jesus referred to himself at multiple occasions as "the son of man", for example Matthew 8:20; 9:6; 10:23; 11:19; 12:8, 40; 13:41; 16:13,27,28; 17:9,12,22; 18:11 (KJV); 19:28; 20:18; 20:28; 24:27,30,37,39,44; 25:13 (KJV),31; 26:2,24,45(KJV),64; Mark 2:10,28; 8:31,38; 9:9,12,31; 10:33,45; 13:26; 14:21,41,62; Luke 5:24; 6:5; 7:34; 9:22,26,44,56(KJV); 9:58(KJV); 11:30; 12:8,10,40; 17:22,24,30; 18:8,31; 19:10; 21:27,36; 22:22,48,69; 24:7; John 1:51; 3:13,14; 5:27; 6:27,53,62; 8:28; 12:23,34; 13:31; Acts 7:56; Revelation 1:13; 14:14.

Trinitarians reason that this prophecy about Jesus mean:

1. Jesus pre-existed as Daniel saw him
2. Jesus is "the son of man" and "the Ancient of Days" which means he is God.
3. When Jesus refer to himself, he refers to the man-part of himself.

This is a bad conclusion because:

1. Prophets like Daniel, often see future events as if it is currently happening. For example see Daniel's other prophecies.
2. In Daniel's vision, the "one like a son of man" was distinct from "the Ancient of Days". They were not the same God.
3. The Jews were expecting a human Messiah (Christ).