Fix regression in decoding large standard calendar times (#5050)

doc/whats-new.rst

@@ -74,6 +74,7 @@ Deprecations
 
 Bug fixes
 ~~~~~~~~~
+- Ensure standard calendar times encoded with large values (i.e. greater than approximately 292 years), can be decoded correctly without silently overflowing (:pull:`5050`).  This was a regression in xarray 0.17.0.  By `Zeb Nicholls <https://github.com/znicholls>`_.
 - Added support for `numpy.bool_` attributes in roundtrips using `h5netcdf` engine with `invalid_netcdf=True` [which casts `bool`s to `numpy.bool_`] (:issue:`4981`, :pull:`4986`).
   By `Victor Negîrneac <https://github.com/caenrigen>`_.
 - Don't allow passing ``axis`` to :py:meth:`Dataset.reduce` methods (:issue:`3510`, :pull:`4940`).

xarray/coding/times.py

@@ -181,6 +181,11 @@ def _decode_datetime_with_pandas(flat_num_dates, units, calendar):
         # strings, in which case we fall back to using cftime
         raise OutOfBoundsDatetime
 
+    with warnings.catch_warnings():
+        warnings.filterwarnings("ignore", "invalid value encountered", RuntimeWarning)
+        pd.to_timedelta(flat_num_dates.min(), delta) + ref_date
+        pd.to_timedelta(flat_num_dates.max(), delta) + ref_date
+
     # To avoid integer overflow when converting to nanosecond units for integer
     # dtypes smaller than np.int64 cast all integer-dtype arrays to np.int64
     # (GH 2002).

xarray/tests/test_coding_times.py

@@ -79,6 +79,9 @@
     (0, "microseconds since 2000-01-01T00:00:00"),
     (np.int32(788961600), "seconds since 1981-01-01"),  # GH2002
     (12300 + np.arange(5), "hour since 1680-01-01 00:00:00.500000"),
+    (164375, "days since 1850-01-01 00:00:00"),
+    (164374.5, "days since 1850-01-01 00:00:00"),
+    ([164374.5, 168360.5], "days since 1850-01-01 00:00:00"),
 ]
 _CF_DATETIME_TESTS = [
     num_dates_units + (calendar,)