---
title: Timestamps
description: Read and send UDX timestamps in RFC 3339 form, with UTC responses and microsecond precision.
---

Every UDX timestamp is an [RFC 3339](https://www.rfc-editor.org/rfc/rfc3339) string. Fields that hold an instant end in `_at`; query parameters that bound an instant end in `_after` or `_before`, and both bounds are exclusive.

## Responses

Responses use UTC with exactly six fractional digits:

```text
2026-06-21T00:00:00.000000Z
```

## Requests

A timestamp in a request must carry an explicit UTC offset and may use zero to six fractional digits. These are all accepted and name the same instant:

```text
2026-01-01T00:00:00Z
2026-01-01T00:00:00.5Z
2025-12-31T19:00:00.000000-05:00
```

A value without an offset, or with more than six fractional digits, returns `400`.

[Request signing](/reference/authentication#sign-with-an-api-key) uses a different clock format: the timestamp header must be Unix time in milliseconds.