Proper Expires Implementation

⇤ Web Application Header Assessment

Required for HTTP/1.0 and optional for HTTP/1.1

The Expires header is required for HTTP/1.0 to indicate when the response gets stale and optional for HTTP/1.1 because Cache-Control (which serves a similar purpose) was only introduced in HTTP/1.1.

Expired headers must either have a value that's an integer less than or equal to zero or contain a valid date in the format specified by RFC-7231. A date more than one year in the future will incur a slight penalty. The date format for the Expires header is defined in RFC-7231 (section 7.1.1.1). The date format strictly specifies GMT as the time zone.

See finding messages.