BUG: np.isnat fails on non-datetime

The function np.isnat asks a very specific question, which is what it says on the tin "is the argument the nan-marker for datetime/timedelta?"

There are situations -- especially in trying to write library code that survives a lot of user inputs (or tests for that) -- where it's necessary or at least desirable to handle NaT specifically.

However the function unfortunately is way less useful than it could be, because it only works for datetime/timedelta:

TypeError: ufunc 'isnat' is only defined for np.datetime64 and np.timedelta64

so now, to use it somewhere (that's not in a hyperspecific chain of branches), I have to write

isinstance(x, (np.datetime64, np.timedelta64)) and np.isnat(x)

to achieve what np.isnat(x) could be doing in the first place.

Why not make this function simply return False instead of TypeError in that case?

Start by locating the np.isnat entry point and examining how it handles non-datetime and non-timedelta inputs. Done means that those inputs return False instead of raising the reported TypeError, while existing datetime and timedelta behavior remains correct.