If-Modified-Since header

The HTTP If-Modified-Since {{Glossary("request header")}}  makes a request conditional. The server sends back the requested resource, with a {{HTTPStatus("200")}}  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 {{HTTPStatus("304")}}  without any body, and the {{HTTPHeader("Last-Modified")}}  response header of the previous request contains the date of the last modification.

Unlike {{HTTPHeader("If-Unmodified-Since")}} , If-Modified-Since can only be used with a {{HTTPMethod("GET")}}  or {{HTTPMethod("HEAD")}} . When used in combination with {{HTTPHeader("If-None-Match")}} , 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 {{HTTPHeader("ETag")}} .

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

{{Specifications}} 

Browser compatibility

{{Compat}} 

See also

• {{HTTPHeader("ETag")}} 
• {{HTTPHeader("If-Match")}} , {{HTTPHeader("If-None-Match")}} , {{HTTPHeader("If-Unmodified-Since")}}  conditional request headers
• {{HTTPStatus("304", "304 Not Modified")}} , {{HTTPStatus("412", "412 Precondition Failed")}}  response status codes