Add compatible timezone support for date_format expression

[andygrove]

Description

The date_format expression was added in PR #3201, but currently only supports UTC timezone with full compatibility. Non-UTC timezones are marked as Incompatible and fall back to Spark by default.

This issue tracks adding proper timezone conversion support so that date_format can be fully compatible with Spark for all timezones.

Current Behavior

• UTC timezone: Compatible() - runs natively in Comet
• Non-UTC timezones: Incompatible() - falls back to Spark by default
• Users can enable non-UTC with spark.comet.expr.DateFormatClass.allowIncompatible=true but results may differ from Spark

Desired Behavior

All timezones should be Compatible() and produce results identical to Spark.

Technical Details

The current implementation uses DataFusion's to_char function which formats timestamps without timezone conversion. Spark's date_format applies the session timezone when formatting.

Possible approaches:

1. Convert the timestamp to the target timezone before calling to_char
2. Use a timezone-aware formatting function if available in DataFusion
3. Implement custom Rust logic to handle timezone conversion

Related

• PR feat: add partial support for date_format expression #3201 - Initial date_format implementation
• Issue date_trunc incorrect results in non-UTC timezone #2649 - Similar timezone issue with date_trunc

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Note: This issue was generated with AI assistance.