Maximilian Linhoff

@byrdie @mhvk It might be worth it to implement separate methods for mean and mean keeping track of rounding errors for maximum precision, as these will have vastly different performance....

It seems this function is used (unintendedly?) to initialize the default values implicitly via `FrameAttribute.getattr`, which is very confusing....

I think you are missing the point. It should be an instance of astropy.time.Time! Just like `np.nan` is an instance of `float`. An instance conveying the meaning that the time...

@taldcroft While the PR you mentioned enables support for masking array time values, there is still no support for a scalar time value that signals missing data or invalid values....

We have hardware failures of a certain telescope component sometimes that results in missing timestamps for certain events. The usecase is simply being able to mark missing/invalid data. `numpy` has...

> At the simplest level do you want to do Time(np.nan)? Yes. > What other operations would be useful? Normal nan propagation. E.g.: * `Time(np.nan) + 5 * u.s` ⇒...

So what exactly is the motivation to allow setting an element of an array `Time` to `nan` but not a single `Time`. That is highly confusing to me.

Also, the error one get's for the second case is very unhelpful: ``` In [20]: Time.now() + np.nan * u.s --------------------------------------------------------------------------- TypeError Traceback (most recent call last) ~/.local/anaconda3/lib/python3.7/site-packages/astropy/units/quantity_helper/converters.py in converters_and_unit(function,...

Yes, this is very similar to our use case, for which I initially opened this issue. I couldn't directly make it work and had to focus on other things for...

I think the main problem is that it requires implementing `__array_ufunc__` for `Time` and that is not simply possible for the one use-case but than has to directly implement all...