The HTTPLast-Modifiedresponse header contains a date and time when the origin server believes the resource was last modified.It is used as a validator inconditional requests (If-Modified-Since orIf-Unmodified-Since) to determine if a requested resource is the same as one already stored by the client.It is less accurate than anETag for determining file contents, but can be used as a fallback mechanism if ETags are unavailable.

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

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

Browser compatibility

See also

• Etag
• HTTP Conditional Requests guide
• If-Match,If-Modified-Since,If-Unmodified-Since,If-None-Match conditional request headers
• 304 Not Modified,412 Precondition Failed response status codes