Dialog is what people say. If you write
"The year is 24356," said Bob.
I read that as
"The year is two four three five six," said Bob.
If you want Bob to say
twenty-four thousand three hundred fifty-six, write it like that.
If you find that awkward to read and write, then maybe your characters would, too, and they wouldn't actually say the year like that. Think of how people say years in real life. Do they always say the full thousands? No. If you asked me, when I was born, I would say something like
nineteen hundred eighty-nine. That kind of relative date is completely sufficient, because you know what century we are in and that I cannot possibly be born in 1889. Or if I were.
Why would the characters in your book behave differently? Their long awkward dates would make it extremely likely that they would use relative dates. Bob, in your example, would say something like
"The year is fifty-six."
When it comes to writing numbers, there is a difference between fiction and non-fiction. The purpose of non-fiction is to convey information. In non-fiction, numbers are written in whichever way they are most easily understood. Therefore, long numbers are written as numerals and structured with dots (25.356).
The purpose of fiction is to depict a world and evoke emotions. In fiction, numbers are written in the way that they appear in the world. A number that a character says is written as a word, because we cannot speak numerals. A number that a character sees written is written as the character sees it:
John followed the hallway until he came to room number four. Or rather, where he thought room number four should be. Because there was a big yellow 5 painted on the fourth door.
"Five," John mused. "Now where is four?"
He turned around. And there he saw it. 4. Number four was on the other side of the hallway.
And a note.
I find it extremely unlikely that people in twenty-five thousand years will still count time following conventions of a long dead culture. Other things will be more relevant to them than the birth of a religious figure they no longer believe in. Just as there have been countless calendars in human history, people in the far future will have their own reckoning of time. In Thailand, the current year is 2560.