DANIEL 1:21
 

And Daniel continued even to the first year of king Cyrus. [AKJV]
 

Thus Daniel continued until the first year of King Cyrus. [Daniel 1:21 NKJV]

Daniel remained politically relevant across five kingdoms because Daniel remained spiritually relevant in them. The spiritual relevancy of Daniel fed or sustained his political relevancy in any kingdom he was. Such a rare gift. Though the four men were found ten times better than all the magicians, enchanters or astrologers, in the entire realms of Babylon, only Daniel among the four remained politically and spiritually relevant through out the entire remaining reign of Nebuchadnezzar, Evilmerodach, Belshazzar, (Kings of Babylon) and Darius (King of Media), and finally Cyrus (King of Persia). And even to the end of the ages, Daniel's prophecies and writings remain relevant over the human race.
 

But Daniel remained indispensable from one kingdom to another, even across five kingdoms physically and spiritually forever, because Daniel had special gifts of God that made him a useful prophet, which his three friends did not have: though they all had the gifts of Wisdom, Skill and understanding Science, that made them ten times better than magicians, astrologers/ enchanters.
 

As things are changing and knowledge increasing, old knowledge and wisdom fade away, and so are their relevance fading off. There are books, knowledge, theories, languages, lessons, etc, that were very relevant in the past but suddenly have no place or relevance in the present or future.
 

Somehow, the political relevance of Hananiah, Mishael and Azariah, (Shadrach, Meshach and Abednego) ended with the reign of Nebuchadnezzar, king of Babylon. But Daniel's political relevance spanned across five successive Kings with changes of empires or overthrowing of world's kingdoms/superpowers. Somewhere, it was Daniel's political request that made the king consider promoting Daniel's three friends: as it is written,
 

[Daniel 2:49] — "Then Daniel requested of the king, and he set Shadrach, Meshach, and Abednego, over the affairs of the province of Babylon: but Daniel sat in the gate of the king."
 

Then later again, after their fire incidence, 
 

[Daniel 3:30] — "Then the king promoted Shadrach, Meshach, and Abednego, in the province of Babylon."
 

Now look again at the top Bible verse above, and notice it mentioned Daniel continued till the first year of Cyrus, king of Persia.
 

Was the first year of Cyrus king of Persia the year that ended Daniel's political relevance? No, seeing by the third year of Cyrus king of Persia was the year the "Prince of Persia" withstood the Angel of God sent to give Daniel understanding. As it is written,
 

[Daniel 10:1] — "In the third year of Cyrus king of Persia a thing was revealed to Daniel, whose name was called Belteshazzar; and the thing was true, but the time appointed was long: and he understood the thing, and had understanding of the vision."
 

So what was the significance of the First Year of Cyrus that made Daniel's political relevance tagged to that year?
 

That first Year of Cyrus was "Beginning of Ezra" or "Beginning of the Seventy Weeks prophetic Calendar" to the coming of Messiah (Daniel 9:24-25).
 

The First Year of Cyrus as Beginning of Ezra:
 

[Ezra 1:1-3] — "In the first year of Cyrus king of Persia, to fulfill the word of the LORD spoken through Jeremiah, the LORD stirred the spirit of Cyrus king of Persia to send a proclamation throughout his kingdom and to put it in writing as follows: This is what Cyrus king of Persia says: “The LORD, the God of heaven, who has given me all the kingdoms of the earth, has appointed me to build a house for Him at Jerusalem in Judah. Whoever among you belongs to His people, may his God be with him, and may he go to Jerusalem in Judah and build the house of the LORD, the God of Israel; He is the God who is in Jerusalem."