Add tests for invalid units and reference date

Author: gcaria

xarray/coding/times.py

@@ -127,6 +127,7 @@ def _unpack_netcdf_time_units(units):
     # whitespace. It also ensures that the year is padded with zeros
     # so it will be correctly understood by pandas (via dateutil).
     matches = re.match(r"(.+) since (.+)", units)
+
     if not matches:
         raise ValueError(f"invalid time units: {units}")
 

xarray/tests/test_coding_times.py

@@ -823,6 +823,12 @@ def test_encode_cf_datetime_overflow(shape):
     roundtrip = decode_cf_datetime(num, units, calendar)
     np.testing.assert_array_equal(dates, roundtrip)
 
+    with pytest.raises(ValueError, match="invalid time units"):
+        encode_cf_datetime(dates, units="days after 2000-01-01")
+
+    with pytest.raises(ValueError, match="invalid reference date"):
+        encode_cf_datetime(dates, units="days since NO_YEAR")
+
 
 def test_encode_cf_datetime_pandas_min():
     # GH 2623