If we want to date something these days, in most countries we use the BC/AD system. So we say something happened in 2021 AD. (Academics now usually use the terms CE/BCE instead.}
However, before the current system was introduced, people would date something from a well known event.
So around 500 years before Jesus, the prophet Daniel wanted to give the exact year that the Messiah would be revealed to the world. (That doesn't mean the year that he was born but the year he started being identified as the messiah in society by his miracles and actions.)
So there was going to be a decree by King Artaxerxes for the Jews to rebuild Jerusalem. Daniel said from that decree it would be 483 years until the Messiah was revealed.
“From the going forth of the command to restore and build Jerusalem until Messiah the Prince, there shall be seven weeks and sixty-two weeks; the street shall be built again, and the wall, even in troublesome times.18
In ancient prophecy, a week meant 7 years. So here Daniel prophesies that from the command to restore Jerusalem to the coming of the Messiah it will be 69 weeks of years, which means 483 years.
The decree went out in 458 BC. 483 years later comes to 25 AD which is the exact year that Jesus was revealed to the world by John the Baptist.
Daniel also says that after that, the Messiah would be killed for the sake of others, prophesying the sacrifical death of Jesus.
“And after the sixty-two weeks Messiah shall be cut off, but not for Himself.”18
This is not some vague prophecy but an absolutely specific one of a specific event on a specific year. And it's proof that the Bible is true.