From 3e293e6ac17e9ebd08aa64330022bbc3aa6ad3d1 Mon Sep 17 00:00:00 2001
From: mottopanikeiku <176798723+mottopanikeiku@users.noreply.github.com>
Date: Tue, 6 Oct 2026 03:34:48 -0700
Subject: [PATCH] Fix timeline bar lengths across daylight saving changes

I calculate timeline bar lengths in the same local wall-clock coordinates used by date axes, so timezone-aware endpoints remain aligned across daylight saving changes. I leave the original start column unchanged and cover spring, autumn, UTC, and naive timestamps across dataframe backends.

Fixes plotly/plotly.py#4611

Signed-off-by: mottopanikeiku <176798723+mottopanikeiku@users.noreply.github.com>
---
 CHANGELOG.md                                  |  1 +
 plotly/express/_core.py                       | 13 +++++++---
 .../test_px/test_px_functions.py              | 26 +++++++++++++++++++
 3 files changed, 37 insertions(+), 3 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index ea837e3d0b..49d1f8f139 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,6 +5,7 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 ## Unreleased
 
 ### Fixed
+- Fix `px.timeline` bars ending an hour early or late when timezone-aware dates span a daylight saving time change [[#4611](https://github.com/plotly/plotly.py/issues/4611)]
 - Fix concurrent first access to lazily initialized graph object properties, which could raise `ValueError("Invalid value")` [[#5691](https://github.com/plotly/plotly.py/pull/5691)], with thanks to @hb1915 for the contribution!
 - Fix `mpl_to_plotly` crashing on touching bars (such as `plt.hist`) due to floating-point noise producing negative `bargap` values by clamping `bargap` to `[0, 1]` [[#5696](https://github.com/plotly/plotly.py/pull/5696)], with thanks to @robertoffmoura for the contribution!
 - Fix `px.sunburst`, `px.treemap` and `px.icicle` listing sectors in a different order on every run when `path` is used with a Polars DataFrame; sectors now follow their order of first appearance for all dataframe backends [[#5766](https://github.com/plotly/plotly.py/pull/5766)], with thanks to @Irahan2 for the contribution!
diff --git a/plotly/express/_core.py b/plotly/express/_core.py
index e99bf40b87..797ffea5f8 100644
--- a/plotly/express/_core.py
+++ b/plotly/express/_core.py
@@ -2193,11 +2193,18 @@ def process_dataframe_timeline(args):
                 "Both x_start and x_end must refer to data convertible to datetimes."
             ) from exc
 
+    # Match the local times displayed on date axes when calculating bar lengths.
+    schema = df.schema
+    x_start, x_end = [
+        nw.col(col).dt.replace_time_zone(None)
+        if schema[col] == nw.Datetime and schema[col].time_zone is not None
+        else nw.col(col)
+        for col in (args["x_start"], args["x_end"])
+    ]
+
     # note that we are not adding any columns to the data frame here, so no risk of overwrite
     args["data_frame"] = df.with_columns(
-        (nw.col(args["x_end"]) - nw.col(args["x_start"]))
-        .dt.total_milliseconds()
-        .alias(args["x_end"])
+        (x_end - x_start).dt.total_milliseconds().alias(args["x_end"])
     )
     args["x"] = args["x_end"]
     args["base"] = args["x_start"]
diff --git a/tests/test_optional/test_px/test_px_functions.py b/tests/test_optional/test_px/test_px_functions.py
index 84a451733b..2ce41c4550 100644
--- a/tests/test_optional/test_px/test_px_functions.py
+++ b/tests/test_optional/test_px/test_px_functions.py
@@ -680,6 +680,32 @@ def test_timeline_cols_already_temporal(constructor, datetime_columns):
     assert fig.layout.xaxis.title.text is None
 
 
+@pytest.mark.parametrize("time_zone", [None, "UTC", "US/Pacific"])
+def test_timeline_daylight_saving(constructor, time_zone):
+    # Regression for https://github.com/plotly/plotly.py/issues/4611
+    starts = ["2024-03-09", "2024-03-10", "2024-03-11", "2023-11-05"]
+    finishes = ["2024-03-10", "2024-03-11", "2024-03-12", "2023-11-06"]
+    df = nw.from_native(
+        constructor({"Start": starts, "Finish": finishes, "Task": ["Job A"] * 4})
+    ).with_columns(
+        nw.col("Start", "Finish")
+        .str.to_datetime(format="%Y-%m-%d")
+        .dt.replace_time_zone(time_zone)
+    )
+
+    fig = px.timeline(df.to_native(), x_start="Start", x_end="Finish", y="Task")
+
+    # Date axes use local wall-clock coordinates: consecutive midnights are
+    # 24 hours apart even when the elapsed time is 23 or 25 hours.
+    assert_array_equal(fig.data[0].x, [24 * 60 * 60 * 1000] * 4)
+    base = np.asarray(fig.data[0].base, dtype="datetime64[ms]")
+    assert_array_equal(base, np.array(starts, dtype="datetime64[ms]"))
+    assert_array_equal(
+        base + fig.data[0].x.astype("timedelta64[ms]"),
+        np.array(finishes, dtype="datetime64[ms]"),
+    )
+
+
 def test_empty_histogram():
     """Empty px.histogram() should not raise, matching scatter/bar/pie behavior.