> In other words, the function responsible for transforming a timestamp into a human-readable date is not injective, as each element of the set of timestamps corresponds to more than one element of the "human dates" set.

The author confuses the concept of injectivity and well-definedness. For an injective function f, it has to be true that given distinct a and b, f(a) != f(b). That's not the problem here; rather the problem is that for a timestamp t there's no unique human-readable date x such that f(t) = x. That's well-definedness, not injectivity.

https://en.wikipedia.org/wiki/Well-defined_expression

You could talk about the inverse function, from human-readable dates to timestamps, and correctly point out that that function is not injective, because more than one human-readable date maps to the same timestamp...