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Applies to:
  • Plan - Any
  • Deployment - Any
  • Use case - Sending timezone-aware datetime fields in trace data and seeing them converted to UTC

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Summary

This article applies to datetime fields stored in trace data, such as input, output, or metadata. It does not describe how the Braintrust UI formats trace timestamps. In the trace Details view, including Metrics > Start, Braintrust displays timestamps in your browser’s local timezone. Hover over Metrics > Start to see both local time and UTC.
Issue: Datetime fields sent with timezone offsets appear converted to UTC in Braintrust traces. For example, 2026-07-03T08:30:00+01:00 is stored and displayed as 2026-07-03T07:30:00Z. Cause: Brainstore automatically normalizes any field that parses as RFC3339 to UTC at index time. Resolution: Store the original UTC offset in a separate metadata field alongside the datetime field.

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Resolution steps

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If you need to preserve the original timezone offset:

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Step 1: Add a separate field for the offset

Store the offset as a plain string in a companion field. This keeps your datetime field functional for filtering and date operators.

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Otherwise, if you want to prevent UTC conversion entirely:

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Step 1: Store the value as a non-parseable string

Insert a space before the offset to cause the datetime cast to fail. Brainstore will then store the value as a plain string.

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Understand the tradeoff

Fields stored as strings are not indexed as datetime. This means:
  • Date operators are unavailable when querying
  • Filtering on this field will be less efficient
This approach is not recommended unless datetime querying is unnecessary for the field.