The HTTP If-Modified-Since makes a request conditional. The server sends back the requested resource, with a status, only if it has been modified after the date in the If-Modified-Since header. If the resource has not been modified since, the response is a without any body, and the response header of the previous request contains the date of the last modification.

Unlike , If-Modified-Since can only be used with a or . When used in combination with , it is ignored, unless the server doesn’t support If-None-Match.

The most common use case is to update a cached entity that has no associated .

Syntax

If-Modified-Since: <day-name>, <day> <month> <year> <hour>:<minute>:<second> GMT

Directives

• <day-name>

  • : One of Mon, Tue, Wed, Thu, Fri, Sat, or Sun (case-sensitive).

  <day>

  • : 2 digit day number, e.g., “04” or “23”.

  <month>

  • : One of Jan, Feb, Mar, Apr, May, Jun, Jul, Aug, Sep, Oct, Nov, Dec (case sensitive).

  <year>

  • : 4 digit year number, e.g., “1990” or “2016”.

  <hour>

  • : 2 digit hour number, e.g., “09” or “23”.

  <minute>

  • : 2 digit minute number, e.g., “04” or “59”.

  <second>

  • : 2 digit second number, e.g., “04” or “59”.

  GMT

  • : Greenwich Mean Time. HTTP dates are always expressed in GMT, never in local time.

Examples

If-Modified-Since: Wed, 21 Oct 2015 07:28:00 GMT

Specifications

Browser compatibility

See also

• , , conditional request headers

• , response status codes