Add tests for invalid units and reference date

Author: gcaria

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([1, 2, 3], units="moments since 2000-01-01")
+
+    with pytest.raises(ValueError, match="invalid reference date"):
+        encode_cf_datetime([1, 2, 3], units="days since NO_YEAR")
+
 
 def test_encode_cf_datetime_pandas_min():
     # GH 2623