> ## Documentation Index
> Fetch the complete documentation index at: https://docs.braintrust.dev/llms.txt
> Use this file to discover all available pages before exploring further.

# Datetime fields in trace data are normalized to UTC

> Understand why Brainstore normalizes timezone-aware datetime fields in trace data to UTC at index time, and how to preserve the original offset.

> [!NOTE]
>
> **Applies to:**
>
> * Plan - Any
> * Deployment - Any
> *
> * Use case - Sending timezone-aware datetime fields in trace data and seeing them converted to UTC

## Summary

> [!NOTE]
>
> 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.

## Resolution steps

### If you need to preserve the original timezone offset:

#### 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.

```json theme={"theme":{"light":"github-light","dark":"github-dark-dimmed"}}
{
  "startTime": "2026-07-03T08:30:00+01:00",
  "startTimeOffset": "+01:00"
}
```

### Otherwise, if you want to prevent UTC conversion entirely:

#### 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.

```json theme={"theme":{"light":"github-light","dark":"github-dark-dimmed"}}
{
  "startTime": "2026-07-03T08:30:00 +01:00"
}
```

#### 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.


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