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Last-Modified header

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

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The HTTP Last-Modified {{glossary("response header")}} contains a date and time when the origin server believes the resource was last modified. It is used as a validator in conditional requests ({{HTTPHeader("If-Modified-Since")}} or {{HTTPHeader("If-Unmodified-Since")}}) to determine if a requested resource is the same as one already stored by the client. It is less accurate than an {{HTTPHeader("ETag")}} for determining file contents, but can be used as a fallback mechanism if ETags are unavailable.

Last-Modified is also used by crawlers to adjust crawl frequency, by browsers in heuristic caching, and by content management systems (CMS) to display the time the content was last modified.

<table class="properties"> <tbody> <tr> <th scope="row">Header type</th> <td>{{glossary("Response header")}}, {{Glossary("Representation header")}}</td> </tr> <tr> <th scope="row">{{Glossary("Forbidden request header")}}</th> <td>No</td> </tr> <tr> <th scope="row"> {{Glossary("CORS-safelisted response header")}} </th> <td>Yes</td> </tr> </tbody> </table>

Syntax

http
Last-Modified: <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
Last-Modified: Wed, 21 Oct 2015 07:28:00 GMT

Specifications

{{Specifications}}

Browser compatibility

{{Compat}}

See also

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