Adapt to CF conventions 1.9 - remove gregorian calendar

HISTORY.rst

@@ -11,6 +11,10 @@ New features and enhancements
 * Added an optimized pathway for ``xclim.indices.run_length`` functions when ``window=1``. (:pull:`911`, :issue:`910`).
 * The data input frequency expected by ``Indicator``s is now in the ``src_freq`` attribute and is thus controlable by subclassing existing indicators. (:issue:`898`, :pull:`927`).
 
+Breaking changes
+~~~~~~~~~~~~~~~~
+* Following version 1.9 of the CF Conventions, publised in September 2021, the calendar name "gregorian" is deprecated. ``core.calendar.get_calendar`` will return "standard", even if the underlying cftime objects still use "gregorian" (cftime <= 1.5.1). (:pull:`935`).
+
 Internal changes
 ~~~~~~~~~~~~~~~~
 * Removed some logging configurations in ``dataflags`` that were polluting python's main logging configuration. (:pull:`909`).

xclim/core/calendar.py

@@ -70,19 +70,23 @@ def get_calendar(obj: Any, dim: str = "time") -> str:
     -------
     str
       The cftime calendar name or "default" when the data is using numpy's or python's datetime types.
+      Will always return "standard" instead of "gregorian", following CF conventions 1.9.
     """
     if isinstance(obj, (xr.DataArray, xr.Dataset)):
         if obj[dim].dtype == "O":
             obj = obj[dim].where(obj[dim].notnull(), drop=True)[0].item()
         elif "datetime64" in obj[dim].dtype.name:
             return "default"
-
-    obj = np.take(
-        obj, 0
-    )  # Take zeroth element, overcome cases when arrays or lists are passed.
+    elif isinstance(obj, xr.CFTimeIndex):
+        obj = obj.values[0]
+    else:
+        obj = np.take(obj, 0)
+        # Take zeroth element, overcome cases when arrays or lists are passed.
     if isinstance(obj, pydt.datetime):  # Also covers pandas Timestamp
         return "default"
     if isinstance(obj, cftime.datetime):
+        if obj.calendar == "gregorian":
+            return "standard"
         return obj.calendar
 
     raise ValueError(f"Calendar could not be inferred from object of type {type(obj)}.")
@@ -450,14 +454,16 @@ def ensure_cftime_array(
 ) -> Union[CFTimeIndex, np.ndarray]:
     """Convert an input 1D array to an array of cftime objects.
 
-    Python's datetime are converted to cftime.DatetimeGregorian.
+    Python's datetime are converted to cftime.DatetimeGregorian ("standard" calendar).
 
     Raises ValueError when unable to cast the input.
     """
     if isinstance(time, xr.DataArray):
         time = time.indexes["time"]
     elif isinstance(time, np.ndarray):
         time = pd.DatetimeIndex(time)
+    if isinstance(time, xr.CFTimeIndex):
+        return time.values
     if isinstance(time[0], cftime.datetime):
         return time
     if isinstance(time[0], pydt.datetime):

xclim/testing/tests/test_calendar.py

@@ -331,7 +331,7 @@ def test_convert_calendar_missing(source, target, freq):
         ("standard", "noleap"),
         ("noleap", "default"),
         ("standard", "360_day"),
-        ("360_day", "gregorian"),
+        ("360_day", "standard"),
         ("noleap", "all_leap"),
         ("360_day", "noleap"),
     ],
@@ -368,20 +368,20 @@ def test_interp_calendar(source, target):
                 dims=("time",),
                 name="time",
             ),
-            "gregorian",
+            "standard",
         ),
-        (date_range("2004-01-01", "2004-01-10", freq="D"), "gregorian"),
+        (date_range("2004-01-01", "2004-01-10", freq="D"), "standard"),
         (
             xr.DataArray(date_range("2004-01-01", "2004-01-10", freq="D")).values,
-            "gregorian",
+            "standard",
         ),
-        (date_range("2004-01-01", "2004-01-10", freq="D"), "gregorian"),
+        (date_range("2004-01-01", "2004-01-10", freq="D"), "standard"),
         (date_range("2004-01-01", "2004-01-10", freq="D", calendar="julian"), "julian"),
     ],
 )
 def test_ensure_cftime_array(inp, calout):
     out = ensure_cftime_array(inp)
-    assert out[0].calendar == calout
+    assert get_calendar(out) == calout
 
 
 @pytest.mark.parametrize(
@@ -391,7 +391,7 @@ def test_ensure_cftime_array(inp, calout):
         (2004, "noleap", 365),
         (2004, "all_leap", 366),
         (1500, "default", 365),
-        (1500, "gregorian", 366),
+        (1500, "standard", 366),
         (1500, "proleptic_gregorian", 365),
         (2030, "360_day", 360),
     ],