ERR: Raise error in to_excel when saving datetimes with timezones

doc/source/whatsnew/v0.25.0.rst

@@ -561,6 +561,7 @@ Other API changes
 - The ``.str``-accessor has been disabled for 1-level :class:`MultiIndex`, use :meth:`MultiIndex.to_flat_index` if necessary (:issue:`23679`)
 - Removed support of gtk package for clipboards (:issue:`26563`)
 - Using an unsupported version of Beautiful Soup 4 will now raise an ``ImportError`` instead of a ``ValueError`` (:issue:`27063`)
+- :meth:`Series.to_excel` and :meth:`DataFrame.to_excel` will now raise a ``ValueError`` when saving timezone aware data. (:issue:`27008`, :issue:`7056`)
 
 .. _whatsnew_0250.deprecations:
 

pandas/io/formats/excel.py

@@ -402,6 +402,10 @@ def _format_value(self, val):
                 val = '-{inf}'.format(inf=self.inf_rep)
             elif self.float_format is not None:
                 val = float(self.float_format % val)
+        if getattr(val, 'tzinfo', None) is not None:
+            raise ValueError('Excel does not support datetimes with '
+                             'timezones. Please ensure that datetimes '
+                             'are timezone unaware before writing to Excel.')
         return val
 
     def _format_header_mi(self):

pandas/tests/io/excel/test_writers.py

@@ -1178,6 +1178,21 @@ def test_merged_cell_custom_objects(self, engine, merge_cells, ext):
         expected.index = expected.index.astype(np.float64)
         tm.assert_frame_equal(expected, result)
 
+    @pytest.mark.parametrize('dtype', [None, object])
+    def test_raise_when_saving_timezones(self, engine, ext, dtype,
+                                         tz_aware_fixture):
+        # GH 27008, GH 7056
+        tz = tz_aware_fixture
+        data = pd.Timestamp('2019', tz=tz)
+        df = DataFrame([data], dtype=dtype)
+        with pytest.raises(ValueError, match="Excel does not support"):
+            df.to_excel(self.path)
+
+        data = data.to_pydatetime()
+        df = DataFrame([data], dtype=dtype)
+        with pytest.raises(ValueError, match="Excel does not support"):
+            df.to_excel(self.path)
+
 
 class TestExcelWriterEngineTests: