What is a Date header?
The date and time at which the message was originated, in HTTP-date format.
Every HTTP response a website sends carries a Date header stamping when it was generated, which caches and clients use to reason about freshness alongside Cache-Control and Expires. It's easy to overlook, but a server with a badly drifted clock can quietly break caching and cookie expiry logic across a site - a subtle failure mode that a monitor watching for unexpected caching or session behaviour over time is more likely to surface than a one-off manual test.
Common use case
Clients compare the response Date against their own clock to detect significant clock drift, and caches use it alongside Age to calculate how stale a stored response has become.
Example
Date: Sun, 23 Aug 2026 10:32:47 GMTHistory
Part of the original HTTP/1.0 specification, RFC 1945 (1996), using the RFC 1123 date format inherited from early internet mail standards.
Did you know?
Despite the many date formats HTTP historically allowed for backwards compatibility, the spec has required this one specific format for any newly generated header since HTTP/1.1.