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If-Modified-Since header

files/en-us/web/http/reference/headers/if-modified-since/index.md

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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")}}.

<table class="properties"> <tbody> <tr> <th scope="row">Header type</th> <td>{{Glossary("Request header")}}</td> </tr> <tr> <th scope="row">{{Glossary("Forbidden request header")}}</th> <td>No</td> </tr> </tbody> </table>

Syntax

http
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

http
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