The countdown
Daniel was an exile in Babylon. Jerusalem lay in ruins. He prayed for his people, and the angel Gabriel came with an answer:
"Seventy weeks are shortened upon thy people, and upon thy holy city, that transgression may be finished, and sin may have an end, and iniquity may be abolished; and everlasting justice may be brought; and vision and prophecy may be fulfilled; and the Saint of saints may be anointed."
Daniel 9:24
A "week" here is a week of years: seven years. Seventy weeks is 490 years.
When the clock starts
"Know thou, therefore, and take notice: that from the going forth of the word, to build up Jerusalem again, unto Christ, the prince, there shall be seven weeks, and sixty-two weeks."
Daniel 9:25
Seven weeks and sixty-two weeks is 69 weeks. 69 times 7 is 483 years, from the order to rebuild Jerusalem to the arrival of the Messiah.
The order came from the Persian king Artaxerxes, in the seventh year of his reign (1 Esdras 7:7 to 13). That was 457 BC.
Where it lands
457 BC plus 483 years is AD 27. There is no year zero, so the count goes straight from 1 BC to AD 1.
Around AD 27, Jesus was baptised in the Jordan and began his public ministry (Luke 3:1 to 23). The Messiah stepped forward right on the date.
Then he is killed
The countdown doesn't end in triumph. It ends in death:
"And after sixty-two weeks Christ shall be slain: and the people that shall deny him shall not be his. And a people, with their leader, that shall come, shall destroy the city, and the sanctuary."
Daniel 9:26
The Douay-Rheims doesn't even leave room to wonder. Christ shall be slain. Then a people and their leader will come and destroy the city and the Temple.
The deadline
In AD 70, the Roman army under Titus destroyed Jerusalem and burned the Temple. It has never been rebuilt.
So the order is fixed. First the Messiah comes. Then he is killed. Then the Temple falls. Anyone claiming to be the Messiah had to arrive, and die, before AD 70.
Jesus was crucified around 40 years before the Temple fell.
Why this is a big deal
Daniel didn't just say the Messiah would come. He gave the year he would appear, said he would be killed, and named the event that would follow.
Only one man fits every line, and he fits them in order.