This fix enables you to preserve the datetime precision when using the melt method. It fixes the issues raised in #55254.

Author: paulreece

doc/source/whatsnew/v2.1.1.rst

@@ -38,6 +38,7 @@ Bug fixes
 - Fixed bug in :meth:`DataFrame.stack` with ``future_stack=True`` and columns a non-:class:`MultiIndex` consisting of tuples (:issue:`54948`)
 - Fixed bug in :meth:`Series.dt.tz` with :class:`ArrowDtype` where a string was returned instead of a ``tzinfo`` object (:issue:`55003`)
 - Fixed bug in :meth:`Series.pct_change` and :meth:`DataFrame.pct_change` showing unnecessary ``FutureWarning`` (:issue:`54981`)
+- Fixed bug where using the melt method would not preserve the datetime (:issue:`55254`)
 
 .. ---------------------------------------------------------------------------
 .. _whatsnew_211.other:

pandas/core/reshape/melt.py

@@ -7,7 +7,10 @@
 
 from pandas.util._decorators import Appender
 
-from pandas.core.dtypes.common import is_list_like
+from pandas.core.dtypes.common import (
+    DatetimeTZDtype,
+    is_list_like,
+)
 from pandas.core.dtypes.concat import concat_compat
 from pandas.core.dtypes.missing import notna
 
@@ -134,7 +137,9 @@ def melt(
 
     mcolumns = id_vars + var_name + [value_name]
 
-    if frame.shape[1] > 0:
+    if frame.shape[1] > 0 and not any(
+        isinstance(dt, DatetimeTZDtype) for dt in frame.dtypes.values
+    ):
         mdata[value_name] = concat(
             [frame.iloc[:, i] for i in range(frame.shape[1])]
         ).values

pandas/tests/reshape/test_melt.py

@@ -459,6 +459,47 @@ def test_melt_ea_columns(self):
         )
         tm.assert_frame_equal(result, expected)
 
+    def test_melt_preserves_datetime(self):
+        df = DataFrame(
+            data=[
+                {
+                    "type": "A0",
+                    "start_date": pd.Timestamp("2023/03/01", tz="Asia/Tokyo"),
+                    "end_date": pd.Timestamp("2023/03/10", tz="Asia/Tokyo"),
+                },
+                {
+                    "type": "A1",
+                    "start_date": pd.Timestamp("2023/03/01", tz="Asia/Tokyo"),
+                    "end_date": pd.Timestamp("2023/03/11", tz="Asia/Tokyo"),
+                },
+            ],
+            index=["aaaa", "bbbb"],
+        )
+        result = df.melt(
+            id_vars=["type"],
+            value_vars=["start_date", "end_date"],
+            var_name="start/end",
+            value_name="date",
+        )
+        expected = DataFrame(
+            {
+                "type": {0: "A0", 1: "A1", 2: "A0", 3: "A1"},
+                "start/end": {
+                    0: "start_date",
+                    1: "start_date",
+                    2: "end_date",
+                    3: "end_date",
+                },
+                "date": {
+                    0: pd.Timestamp("2023-03-01 00:00:00+0900", tz="Asia/Tokyo"),
+                    1: pd.Timestamp("2023-03-01 00:00:00+0900", tz="Asia/Tokyo"),
+                    2: pd.Timestamp("2023-03-10 00:00:00+0900", tz="Asia/Tokyo"),
+                    3: pd.Timestamp("2023-03-11 00:00:00+0900", tz="Asia/Tokyo"),
+                },
+            }
+        )
+        tm.assert_frame_equal(result, expected)
+
 
 class TestLreshape:
     def test_pairs(self):