date.c: Remove all occurrences of g_return_val_if_fail

That's got to be one of the hardest macro names to read, ever,
(it's phrased with an implicit negative in the condition,
rather than something simple like "assert").

Plus, it's evil, since it's a macro with a return in it.

And finally, it's actually *longer* than just typing "if"
and "return". So what's the point of this ugly idiom?