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Last-Modified header

The HTTP Last-Modified {{glossary("response header")}}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")}}Since or {{HTTPHeader("If-Unmodified-Since")}}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")}}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.

Header type {{glossary("[Response header")}}header](https://developer.mozilla.org/en-US/docs/Glossary/Response%20header), {{Glossary("[Representation header")}}header](https://developer.mozilla.org/en-US/docs/Glossary/Representation%20header)
{{Glossary("[Forbidden request header")}}header](https://developer.mozilla.org/en-US/docs/Glossary/Forbidden%20request%20header) No
{{Glossary("[CORS-safelisted response header")}}header](https://developer.mozilla.org/en-US/docs/Glossary/CORS-safelisted%20response%20header) Yes

Syntax

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

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

Specifications

{{Specifications}}

Browser compatibility

{{Compat}}

See also