issues: 339215679

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id node_id number title user state locked assignee milestone comments created_at updated_at closed_at author_association active_lock_reason draft pull_request body reactions performed_via_github_app state_reason repo type
339215679 MDU6SXNzdWUzMzkyMTU2Nzk= 2272 [bug ] xarry np.timedelta64 overflow 12111171 closed 0     0 2018-07-08T10:43:47Z 2018-10-28T00:38:59Z 2018-10-28T00:38:59Z NONE      

```python import numpy as np import xarray as xr

ds = xr.Dataset(coords={'time': ( 'time', np.arange(106300.5, 106665.5+5*365, 365), {'units': 'days since 1800-01-01 00:00:00'})}) ds = xr.decode_cf(ds) ds.to_netcdf('./test.nc') ds = xr.open_dataset('./test.nc', decode_cf=False) print(ds.time)

<xarray.DataArray 'time' (time: 6)> array([ 106300.5 , 106665.5 , -106473.482335, -106108.482335, -105743.482335, -105378.482335]) Coordinates: * time (time) float64 1.063e+05 1.067e+05 -1.065e+05 -1.061e+05 ... Attributes: _FillValue: nan units: days since 1800-01-01 calendar: proleptic_gregorian ```

Problem description

The saved netCDF file contains negative time values since "292 years is the maximum length of time a np.timedelta64 object with nanosecond precision can represent" (see here in the documentation) and it therefore runs into an overflow in the example (via skcs explanation to my question on StackOverflow).

Expected Output

Correct time values.

Possible solution:

For dates outside of the np.datetime range the behaviour is already correct since xarray falls back to cftime.datetime. I don't know if this could be a feasible solution but I wanted to mention it here.

```python import numpy as np import xarray as xr

ds = xr.Dataset(coords={'time': ( 'time', np.arange(106300.5, 106665.5+5*365, 365), {'units': 'days since 0001-01-01 00:00:00'})})

ds = xr.decode_cf(ds)

/opt/anaconda2/envs/py36/lib/python3.6/site-packages/xarray/coding/times.py:132: SerializationWarning: Unable to decode time axis into full numpy.datetime64 objects, continuing using dummy cftime.datetime objects instead, reason: dates out of range enable_cftimeindex) /opt/anaconda2/envs/py36/lib/python3.6/site-packages/xarray/coding/variables.py:66: SerializationWarning: Unable to decode time axis into full numpy.datetime64 objects, continuing using dummy cftime.datetime objects instead, reason: dates out of range return self.func(self.array[key])

ds.to_netcdf('./test.nc') ds = xr.open_dataset('./test.nc', decode_cf=False) print(ds.time)

<xarray.DataArray 'time' (time: 6)> array([106300.5, 106665.5, 107030.5, 107395.5, 107760.5, 108125.5]) Coordinates: * time (time) float64 1.063e+05 1.067e+05 1.07e+05 1.074e+05 1.078e+05 ... Attributes: _FillValue: nan units: days since 0001-01-01 calendar: gregorian ```

Output of xr.show_versions()

INSTALLED VERSIONS ------------------ commit: None python: 3.6.4.final.0 python-bits: 64 OS: Linux OS-release: 4.15.0-24-generic machine: x86_64 processor: x86_64 byteorder: little LC_ALL: None LANG: en_GB.UTF-8 LOCALE: en_GB.UTF-8 xarray: 0.10.7 pandas: 0.22.0 numpy: 1.14.2 scipy: 1.0.1 netCDF4: 1.3.1 h5netcdf: 0.6.1 h5py: 2.8.0 Nio: None zarr: None bottleneck: 1.2.1 cyordereddict: None dask: 0.18.1 distributed: 1.22.0 matplotlib: 2.2.2 cartopy: 0.16.0 seaborn: 0.8.1 setuptools: 39.0.1 pip: 9.0.1 conda: None pytest: None IPython: 6.2.1 sphinx: 1.7.2 
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   completed 13221727 issue

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