issue_comments: 784497022

I got a similar error message when opening a Zarr store with datetime64 time values, where I tried to set the "calendar" attribute on the time axis (the attribute was unset in the original store). I've found some xarray code that appears to treat the "calendar" and "units" time attributes as special, and it essentially prevents users from setting or changing these values, even when those values are not present:

From xarray/coding/times.py:

``` class CFDatetimeCoder(VariableCoder): def init(self, use_cftime=None): self.use_cftime = use_cftime

def encode(self, variable, name=None):
    dims, data, attrs, encoding = unpack_for_encoding(variable)
    if np.issubdtype(data.dtype, np.datetime64) or contains_cftime_datetimes(
        variable
    ):
        (data, units, calendar) = encode_cf_datetime(
            data, encoding.pop("units", None), encoding.pop("calendar", None)
        )
        safe_setitem(attrs, "units", units, name=name)
        safe_setitem(attrs, "calendar", calendar, name=name)

    return Variable(dims, data, attrs, encoding)

```