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Unexpected behaviour when casting time-zone aware datetime to date #22864

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svengiegerich opened this issue May 21, 2025 · 0 comments · May be fixed by #22888
Open
2 tasks done

Unexpected behaviour when casting time-zone aware datetime to date #22864

svengiegerich opened this issue May 21, 2025 · 0 comments · May be fixed by #22888
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bug Something isn't working needs triage Awaiting prioritization by a maintainer python Related to Python Polars

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@svengiegerich
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Checks

  • I have checked that this issue has not already been reported.
  • I have confirmed this bug exists on the latest version of Polars.

Reproducible example

from datetime import datetime
from zoneinfo import ZoneInfo

import polars as pl

df = pl.DataFrame({"start": datetime(2021, 1, 1, tzinfo=ZoneInfo("Europe/Berlin"))})

# Correct date (2021-01-01)
print(df.select(pl.col("start").dt.date()).item())

# Incorrect date (2020-12-31)
print(df.select(pl.col("start").cast(pl.Date)).item())

Log output

2021-01-01
2020-12-31

Issue description

When casting a time-zone aware datetime column, the result differs from calling polars.Expr.dt.date().

Expected behavior

I would have expected that the time zone would be respected when casting—i.e., both operations return the same value.

Installed versions

--------Version info---------
Polars:              1.27.1
Index type:          UInt32
Platform:            macOS-15.4-arm64-arm-64bit
Python:              3.12.10 | packaged by conda-forge | (main, Apr 10 2025, 22:19:24) [Clang 18.1.8 ]
LTS CPU:             False

----Optional dependencies----
Azure CLI            <not installed>
adbc_driver_manager  <not installed>
altair               <not installed>
azure.identity       <not installed>
boto3                1.37.1
cloudpickle          3.1.1
connectorx           <not installed>
deltalake            <not installed>
fastexcel            0.13.0
fsspec               2025.3.2
gevent               <not installed>
google.auth          2.39.0
great_tables         <not installed>
matplotlib           3.10.1
numpy                1.26.4
openpyxl             <not installed>
pandas               2.2.3
polars_cloud         <not installed>
pyarrow              19.0.1
pydantic             2.11.3
pyiceberg            0.9.0
sqlalchemy           2.0.40
torch                <not installed>
xlsx2csv             <not installed>
xlsxwriter           <not installed>```

</details>
@svengiegerich svengiegerich added bug Something isn't working python Related to Python Polars needs triage Awaiting prioritization by a maintainer labels May 21, 2025
@svengiegerich svengiegerich mentioned this issue May 21, 2025
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@florian-klein florian-klein linked a pull request May 22, 2025 that will close this issue
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fix(rust): Make casting to date timezone-aware florian-klein/polars
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