Disallow standard calendar np.datetime64 encoding prior to reform

doc/whats-new.rst

@@ -33,7 +33,6 @@ New Features
 Breaking changes
 ~~~~~~~~~~~~~~~~
 
-
 Deprecations
 ~~~~~~~~~~~~
 
@@ -51,6 +50,14 @@ Bug fixes
   calculating mean in rolling for correct operations (preserve float dtypes,
   correct mean of bool arrays) (:issue:`10340`, :pull:`10341`).
   By `Kai Mühlbauer <https://github.com/kmuehlbauer>`_.
+- Raise an error when attempting to encode :py:class:`numpy.datetime64` values
+  prior to the Gregorian calendar reform date of 1582-10-15 with a
+  ``"standard"`` or ``"gregorian"`` calendar. Previously we would warn and
+  encode these as :py:class:`cftime.DatetimeGregorian` objects, but it is not
+  clear that this is the user's intent, since this implicitly converts the
+  calendar of the datetimes from ``"proleptic_gregorian"`` to ``"gregorian"``
+  and prevents round-tripping them as :py:class:`numpy.datetime64` values
+  (:pull:`10352`). By `Spencer Clark <https://github.com/spencerkclark>`_.
 
 Documentation
 ~~~~~~~~~~~~~

xarray/coding/times.py

@@ -1065,6 +1065,7 @@ def _eagerly_encode_cf_datetime(
         calendar = infer_calendar_name(dates)
 
     raise_incompatible_units_error = False
+    raise_gregorian_proleptic_gregorian_mismatch_error = False
     try:
         if not _is_standard_calendar(calendar) or dates.dtype.kind == "O":
             # parse with cftime instead
@@ -1073,16 +1074,7 @@ def _eagerly_encode_cf_datetime(
         if calendar in ["standard", "gregorian"] and np.nanmin(dates).astype(
             "=M8[us]"
         ).astype(datetime) < datetime(1582, 10, 15):
-            # if we use standard calendar and for dates before the reform
-            # we need to use cftime instead
-            emit_user_level_warning(
-                f"Unable to encode numpy.datetime64 objects with {calendar} calendar."
-                "Using cftime.datetime objects instead, reason: dates prior "
-                "reform date (1582-10-15). To silence this warning transform "
-                "numpy.datetime64 to corresponding cftime.datetime beforehand.",
-                SerializationWarning,
-            )
-            raise OutOfBoundsDatetime
+            raise_gregorian_proleptic_gregorian_mismatch_error = True
 
         time_unit, ref_date = _unpack_time_unit_and_ref_date(units)
         # calendar equivalence only for days after the reform
@@ -1166,6 +1158,16 @@ def _eagerly_encode_cf_datetime(
             f"units {units!r}. Consider setting encoding['units'] to {new_units!r} to "
             f"serialize with an integer dtype."
         )
+    if raise_gregorian_proleptic_gregorian_mismatch_error:
+        raise ValueError(
+            f"Unable to encode np.datetime64 values with {calendar} "
+            f"calendar, because some or all values are prior to the reform "
+            f"date of 1582-10-15. To encode these times, set "
+            f"encoding['calendar'] to 'proleptic_gregorian' instead, which "
+            f"is the true calendar that np.datetime64 values use. The "
+            f"'standard' or 'gregorian' calendar is only equivalent to the "
+            f"'proleptic_gregorian' calendar after the reform date."
+        )
 
     return num, units, calendar
 

xarray/tests/test_coding_times.py

@@ -239,8 +239,6 @@ def test_decode_non_standard_calendar_inside_timestamp_range(calendar) -> None:
 def test_decode_dates_outside_timestamp_range(
     calendar, time_unit: PDDatetimeUnitOptions
 ) -> None:
-    from datetime import datetime
-
     import cftime
 
     units = "days since 0001-01-01"
@@ -379,8 +377,6 @@ def test_decode_nonstandard_calendar_multidim_time_inside_timestamp_range(
 def test_decode_multidim_time_outside_timestamp_range(
     calendar, time_unit: PDDatetimeUnitOptions
 ) -> None:
-    from datetime import datetime
-
     import cftime
 
     units = "days since 0001-01-01"
@@ -1163,27 +1159,26 @@ def test__encode_datetime_with_cftime() -> None:
 
 
 @requires_cftime
-def test_encode_decode_cf_datetime_outofbounds_warnings(
+def test_round_trip_standard_calendar_cftime_datetimes_pre_reform() -> None:
+    from cftime import DatetimeGregorian
+
+    dates = np.array([DatetimeGregorian(1, 1, 1), DatetimeGregorian(2000, 1, 1)])
+    encoded = encode_cf_datetime(dates, "seconds since 2000-01-01", "standard")
+    with pytest.warns(SerializationWarning, match="Unable to decode time axis"):
+        decoded = decode_cf_datetime(*encoded)
+    np.testing.assert_equal(decoded, dates)
+
+
+@pytest.mark.parametrize("calendar", ["standard", "gregorian"])
+def test_encode_cf_datetime_gregorian_proleptic_gregorian_mismatch_error(
+    calendar: str,
     time_unit: PDDatetimeUnitOptions,
 ) -> None:
-    import cftime
-
     if time_unit == "ns":
-        pytest.skip("does not work work out of bounds datetimes")
+        pytest.skip("datetime64[ns] values can only be defined post reform")
     dates = np.array(["0001-01-01", "2001-01-01"], dtype=f"datetime64[{time_unit}]")
-    cfdates = np.array(
-        [
-            cftime.datetime(t0.year, t0.month, t0.day, calendar="gregorian")
-            for t0 in dates.astype(datetime)
-        ]
-    )
-    with pytest.warns(
-        SerializationWarning, match="Unable to encode numpy.datetime64 objects"
-    ):
-        encoded = encode_cf_datetime(dates, "seconds since 2000-01-01", "standard")
-    with pytest.warns(SerializationWarning, match="Unable to decode time axis"):
-        decoded = decode_cf_datetime(*encoded)
-    np.testing.assert_equal(decoded, cfdates)
+    with pytest.raises(ValueError, match="proleptic_gregorian"):
+        encode_cf_datetime(dates, "seconds since 2000-01-01", calendar)
 
 
 @pytest.mark.parametrize("calendar", ["gregorian", "Gregorian", "GREGORIAN"])