That number could be anything, such as a user ID. Even among timestamp libraries that work with January 1st 1970 UTC as the zero point, they might parse it as J

That number could be anything, such as a user ID. Even among timestamp libraries that work with January 1st 1970 UTC as the zero point, they might parse it as January 21st 1970 if they were expecting milliseconds rather than seconds. It doesn't get much more ambiguous than a plain number!