BUG: ensure consistent behavior of Index.is_all_dates (pandas-dev#19204)

Author: adam

doc/source/whatsnew/v1.1.0.rst

@@ -479,6 +479,7 @@ Indexing
 - Bug in :meth:`DataFrame.copy` _item_cache not invalidated after copy causes post-copy value updates to not be reflected (:issue:`31784`)
 - Bug in `Series.__getitem__` with an integer key and a :class:`MultiIndex` with leading integer level failing to raise ``KeyError`` if the key is not present in the first level (:issue:`33355`)
 - Bug in :meth:`DataFrame.iloc` when slicing a single column-:class:`DataFrame`` with ``ExtensionDtype`` (e.g. ``df.iloc[:, :1]``) returning an invalid result (:issue:`32957`)
+- Bug in :meth:`Index.is_all_dates` incorrectly returning ``False`` when inferred type of index was other dates than datetime (:issue:`19204`)
 
 Missing
 ^^^^^^^

pandas/core/indexes/base.py

@@ -1989,7 +1989,14 @@ def is_all_dates(self) -> bool:
         """
         Whether or not the index values only consist of dates.
         """
-        return is_datetime_array(ensure_object(self.values))
+        return self.inferred_type in [
+            "datetime64",
+            "datetime",
+            "date",
+            "timedelta64",
+            "timedelta",
+            "period",
+        ]
 
     # --------------------------------------------------------------------
     # Pickle Methods

pandas/tests/indexes/test_base.py

@@ -1159,6 +1159,16 @@ def test_is_object(self, indices, expected):
     def test_is_all_dates(self, indices, expected):
         assert indices.is_all_dates is expected
 
+    @pytest.mark.parametrize(
+        "index",
+        ["datetime", "datetime-tz", "period", "timedelta", "empty"],
+        indirect=["index"],
+    )
+    def test_is_all_dates_consistency(self, index):
+        # GH 19204
+        non_date = pd.Index(["not a date"])
+        assert index.is_all_dates == index.append(non_date)[:-1].is_all_dates
+
     def test_summary(self, indices):
         self._check_method_works(Index._summary, indices)