How is referential transparency enforced?

Question

In FP languages, calling a function with the same parameters over and over again returns the same result over and over again (i.e. referential transparency).

But a function like this (pseudo-code):

function f(a, b) {
    return a + b + currentDateTime.seconds;
}

is not going to return the same result for the same parameters.

How are these cases handled in FP?

How is referential transparency enforced? Or is it not and it depends on the programmers to behave themselves?