Fix non-standard calendars

test/test_conventions.py

@@ -110,6 +110,30 @@ def test_cf_datetime(self):
                         pd.Index(actual), units, calendar)
                     self.assertArrayEqual(num_dates, np.around(encoded))
 
+    @requires_netCDF4
+    def test_decoded_cf_datetime_array(self):
+        import netCDF4 as nc4
+
+        actual = conventions.DecodedCFDatetimeArray(
+            [0, 1, 2], 'days since 1900-01-01', 'standard')
+        expected = pd.date_range('1900-01-01', periods=3).values
+        self.assertEqual(actual.dtype, np.dtype('datetime64[ns]'))
+        self.assertArrayEqual(actual, expected)
+
+        # default calendar
+        actual = conventions.DecodedCFDatetimeArray(
+            [0, 1, 2], 'days since 1900-01-01')
+        self.assertEqual(actual.dtype, np.dtype('datetime64[ns]'))
+        self.assertArrayEqual(actual, expected)
+
+        num_dates = [722000, 720000.5]
+        units = 'days since 0001-01-01 0:0:0'
+        calendar = 'noleap'
+        actual = conventions.DecodedCFDatetimeArray(num_dates, units, calendar)
+        expected = nc4.num2date(num_dates, units, calendar)
+        self.assertEqual(actual.dtype, np.dtype('O'))
+        self.assertArrayEqual(actual, expected)
+
     @requires_netCDF4
     def test_cf_datetime_nan(self):
         for num_dates, units, expected_list in [

xray/conventions.py

@@ -24,6 +24,9 @@
 # coerced instead as indicated by the "coerce_nc3_dtype" function
 _nc3_dtype_coercions = {'int64': 'int32', 'float64': 'float32', 'bool': 'int8'}
 
+# standard calendars recognized by netcdftime
+_STANDARD_CALENDARS = {'standard', 'gregorian', 'proleptic_gregorian'}
+
 
 def coerce_nc3_dtype(arr):
     """Coerce an array to a data type that can be stored in a netCDF-3 file
@@ -163,7 +166,7 @@ def nan_safe_num2date(num):
     else:
         max_date = min_date
 
-    if (calendar not in ['standard', 'gregorian', 'proleptic_gregorian']
+    if (calendar not in _STANDARD_CALENDARS
             or min_date < datetime(1678, 1, 1)
             or max_date > datetime(2262, 4, 11)):
         dates = nc4.num2date(num_dates, units, calendar)
@@ -324,7 +327,13 @@ def __init__(self, array, units, calendar=None):
 
     @property
     def dtype(self):
-        return np.dtype('datetime64[ns]')
+        if self.calendar is None or self.calendar in _STANDARD_CALENDARS:
+            # TODO: return the proper dtype (object) for a standard calendar
+            # that can't be expressed in ns precision. Perhaps we could guess
+            # this from the units?
+            return np.dtype('datetime64[ns]')
+        else:
+            return np.dtype('O')
 
     def __getitem__(self, key):
         return decode_cf_datetime(self.array, units=self.units,