issues: 595492608

Time dtype encoding defaults to "int64" for datasets with only zero-hour times when writing to netcdf or zarr.

This results in these datasets having a precision constrained by how the time units are defined (in the example below daily precision, given units are defined as 'days since ...'). If we for instance create a zarr dataset using this default encoding option with such datasets, and subsequently append some non-zero times onto it, we loose the hour/minute/sec information from the appended bits.

MCVE Code Sample

```python In [1]: ds = xr.DataArray( ...: data=[0.5], ...: coords={"time": [datetime.datetime(2012,1,1)]}, ...: dims=("time",), ...: name="x", ...: ).to_dataset()

In [2]: ds
Out[2]: <xarray.Dataset> Dimensions: (time: 1) Coordinates: * time (time) datetime64[ns] 2012-01-01 Data variables: x (time) float64 0.5

In [3]: ds.to_zarr("/tmp/x.zarr")

In [4]: ds1 = xr.open_zarr("/tmp/x.zarr")

In [5]: ds1.time.encoding
Out[5]: {'chunks': (1,), 'compressor': Blosc(cname='lz4', clevel=5, shuffle=SHUFFLE, blocksize=0), 'filters': None, 'units': 'days since 2012-01-01 00:00:00', 'calendar': 'proleptic_gregorian', 'dtype': dtype('int64')}

In [6]: dsnew = xr.DataArray( ...: data=[1.5], ...: coords={"time": [datetime.datetime(2012,1,1,3,0,0)]}, ...: dims=("time",), ...: name="x", ...: ).to_dataset()

In [7]: dsnew.to_zarr("/tmp/x.zarr", append_dim="time")

In [8]: ds1 = xr.open_zarr("/tmp/x.zarr")

In [9]: ds1.time.values
Out[9]: array(['2012-01-01T00:00:00.000000000', '2012-01-01T00:00:00.000000000'], dtype='datetime64[ns]')

```

Expected Output

In [9]: ds1.time.values Out[9]: array(['2012-01-01T00:00:00.000000000', '2012-01-01T03:00:00.000000000'], dtype='datetime64[ns]')

Problem Description

Perhaps it would be useful defaulting time dtype to "float64". Another option could be using a finer time resolution by default than that automatically defined from xarray based on the dataset times (for instance, if the units are automatically defined as "days since ...", use "seconds since...".

```

Versions

Output of `xr.show_versions()`

In [10]: xr.show_versions() INSTALLED VERSIONS ------------------ commit: None python: 3.7.5 (default, Nov 20 2019, 09:21:52) [GCC 9.2.1 20191008] python-bits: 64 OS: Linux OS-release: 5.3.0-45-generic machine: x86_64 processor: x86_64 byteorder: little LC_ALL: None LANG: en_NZ.UTF-8 LOCALE: en_NZ.UTF-8 libhdf5: 1.10.4 libnetcdf: 4.6.3 xarray: 0.15.0 pandas: 1.0.1 numpy: 1.18.1 scipy: 1.4.1 netCDF4: 1.5.3 pydap: None h5netcdf: 0.8.0 h5py: 2.10.0 Nio: None zarr: 2.4.0 cftime: 1.1.0 nc_time_axis: None PseudoNetCDF: None rasterio: 1.1.3 cfgrib: None iris: None bottleneck: None dask: 2.14.0 distributed: 2.12.0 matplotlib: 3.2.0 cartopy: 0.17.0 seaborn: None numbagg: None setuptools: 45.3.0 pip: 20.0.2 conda: None pytest: 5.3.5 IPython: 7.13.0 sphinx: None