Gantt chart export with kaleido v1.0.0 shows 1970 epoch #392

Problem Description

I'm reporting a bug with the new kaleido v1.0.0 engine, when I make a Gantt chart with plotly.express.timeline. The issue is how it handles my datetime objects (and with sting date) when I try to save a static image.

I found the problem in a couple of ways:

1. When I run fig.show() in my Jupyter Notebook, the chart looks perfect with the right 2025 dates. But when I save that same figure with fig.write_image(), I get a broken image. Its x-axis starts at 1970.
2. I get the exact same broken image with 1970 dates when I run my code as a normal Python script.

It really makes me think this is a regression bug in the kaleido v1.0.0 renderer.

Minimal Reproducible Example

This script will always create the error. The /// script part is there for anyone using a tool like Astral's uv.

# /// script
# requires-python = "==3.13.1"
# dependencies = [
#     "pandas==2.3.1",
#     "plotly==6.3.0",
#     "kaleido==1.0.0",
# ]
# ///

import pandas as pd
import plotly.express as px

data: list[dict] = [
    dict(Start="2025-08-16", Finish="2025-08-30", Task="Data Collection"),
    dict(Start="2025-08-25", Finish="2025-09-15", Task="Identifying Problem"),
    dict(Start="2025-09-13", Finish="2025-09-30", Task="Review & Submission"),
]

df: pd.DataFrame = pd.DataFrame(data)
df.Start = pd.to_datetime(df.Start)
df.Finish = pd.to_datetime(df.Finish)

fig = px.timeline(df, x_start="Start", x_end="Finish", y="Task", color="Task")
fig.update_layout(template="presentation")
fig.update_layout(autosize=False, width=1000, height=300, margin=dict(l=270, r=10))

fig.update_layout(
    showlegend=False,
    title="Gantt Chart",
    bargap=0.45,
)

# This makes the wrong image
fig.write_image("Gantt_Chart.png")

Expected Behavior

I expected my saved file Gantt_Chart.png to show a chart with an x-axis properly showing dates in August and September 2025.

Actual Behavior

What I actually got in Gantt_Chart.png was a chart where the x-axis starts near January 5, 1970.

Working Environment Confirmation

I ran a control test to be sure. I tested my same code in an environment with older libraries:

• plotly==6.0.0
• kaleido==0.2.1

In that setup, the bug is gone. My list of confirmations show that the static image Gantt_Chart.png saves perfect with the right 2025 dates.

This points me to think the problem is a regression in the newer code.

Failing Environment

• Python: 3.13.1
• pandas: 2.3.1
• plotly: 6.3.0
• kaleido: 1.0.0