Date

What is the Date header?

The Date header, defined in Section 6.6.1 of RFC 9110, gives the date and time at which a message was originated. Its value is an HTTP-date, and every sender must generate that in the IMF-fixdate format:

Date: Tue, 15 Nov 1994 08:12:31 GMT

IMF-fixdate is fixed-length, always UTC, and always written with the literal GMT. The day and month names are case-sensitive three-letter English abbreviations, the day is two digits, and there is no extra whitespace. Recipients must also accept two obsolete formats (the RFC 850 format and C’s asctime()), but nothing may generate them.

An origin server with a clock must send Date on every 2xx, 3xx and 4xx response, and may send it on 1xx and 5xx. An origin without a reliable clock must not send it at all. A cache or proxy with a clock that receives a response without Date must add one if it caches or forwards the response.

API Usage & Best Practices

• Let your server or framework set it. Almost every HTTP server emits a correct Date automatically. Hand-written Date values are where non-compliant formats come from.
• Never send ISO 8601 in it. Date: 2026-10-14T08:12:31Z is not an HTTP-date, and a strict parser will reject it. ISO 8601 belongs in your JSON bodies; see timestamps and formats. The same rule applies to every header that takes an HTTP-date: Last-Modified, Expires, Retry-After and Sunset.
• Keep the clock in sync. RFC 9110 expects a clock to be synchronized with NTP or similar. Caches use Date to calculate the apparent age of a response, together with Age, so a skewed origin clock makes responses look older or younger than they are.
• Use it to detect client clock skew. A client that signs requests with a timestamp, or validates token expiry locally, can compare its own clock against the Date on a response and correct for the difference rather than failing mysteriously.
• Do not rely on it for ordering events. Date has one-second resolution and reflects one server’s clock. For ordering resources, use version fields or ETag, not response dates.
• Clients rarely need to send it. A user agent may include Date in a request, but normally only does so when a scheme specifically asks for it, such as a request-signing protocol that covers the header.

Examples

A response with a correctly formatted Date alongside other HTTP-date fields:

HTTP/1.1 200 OK
Date: Wed, 14 Oct 2026 08:12:31 GMT
Last-Modified: Tue, 13 Oct 2026 17:40:02 GMT
Cache-Control: max-age=300
Content-Type: application/json

Formats that are not valid for a sender to generate:

Date: 2026-10-14T08:12:31Z            ISO 8601, not an HTTP-date
Date: Wed, 14 Oct 2026 08:12:31 UTC   must be GMT
Date: wed, 14 oct 2026 08:12:31 GMT   names are case-sensitive
Date: Wed, 14 Oct 2026 8:12:31 GMT    hour must be two digits