BUG: Fix to_html adding time to datetime columns in transposed DataFrame (GH#10640)

Author: epabarker

doc/source/whatsnew/v3.0.0.rst

@@ -1290,6 +1290,7 @@ I/O
 - Bug in :meth:`read_csv` with ``engine="pyarrow"`` and ``dtype="Int64"`` losing precision (:issue:`56136`)
 - Bug in :meth:`read_excel` raising ``ValueError`` when passing array of boolean values when ``dtype="boolean"``. (:issue:`58159`)
 - Bug in :meth:`read_html` where ``rowspan`` in header row causes incorrect conversion to ``DataFrame``. (:issue:`60210`)
+- Bug in :meth:`DataFrame.to_html` where a :class:`DatetimeIndex` in the columns was including the time even when it was 00:00:00 (:issue:`10640`)
 - Bug in :meth:`read_json` ignoring the given ``dtype`` when ``engine="pyarrow"`` (:issue:`59516`)
 - Bug in :meth:`read_json` not validating the ``typ`` argument to not be exactly ``"frame"`` or ``"series"`` (:issue:`59124`)
 - Bug in :meth:`read_json` where extreme value integers in string format were incorrectly parsed as a different integer number (:issue:`20608`)

pandas/io/formats/html.py

@@ -130,7 +130,7 @@ def row_levels(self) -> int:
         return 0
 
     def _get_columns_formatted_values(self) -> Iterable:
-        return self.columns
+        return self.columns._format_flat(include_name=False)
 
     @property
     def is_truncated(self) -> bool:

pandas/tests/io/formats/test_to_html.py

@@ -1165,3 +1165,18 @@ def test_to_html_empty_complex_array():
         "</table>"
     )
     assert result == expected
+
+
+def test_to_html_datetime_columns_no_time():
+    # GH 10640
+    df = DataFrame(
+        np.random.default_rng(2).standard_normal((5, 2)),
+        columns=["a", "b"],
+        index=pd.to_datetime(
+            ["2015-01-05", "2015-01-04", "2015-01-03", "2015-01-02", "2015-01-01"]
+        ),
+    )
+    result = df.T.to_html()
+    assert "2015-01-05 00:00:00" not in result
+    assert "2015-01-05" in result
+