There's a lot more context baked into an ISO string than a unix timestamp though. Write a regex to find all the strict ISO8601-RFC3339 strings in a directory. Now write one for unix timestamps.

> Also calculating amount of time between two ISO8601 strings without libraries is nor trivial, or any other operation actually.

That's true of any datetime that actual humans use (as opposed to computers, financial systems, sysadmins and programmers).