BUG: fixes indexing with monotonic decreasing DTI (pandas-dev#19362)

Author: mapehe

pandas/core/indexes/datetimelike.py

@@ -342,7 +342,7 @@ def _format_with_header(self, header, **kwargs):
     def __contains__(self, key):
         try:
             res = self.get_loc(key)
-            return is_scalar(res) or type(res) == slice or np.any(res)
+            return is_scalar(res) or type(res) == slice or np.any(res+1)
         except (KeyError, TypeError, ValueError):
             return False