Last-Modified
The HTTP Last-Modified
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 (If-Modified-Since
or 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 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 | Response header, Representation header |
---|---|
Forbidden header name | No |
CORS-safelisted response header | 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
Specification |
---|
HTTP Semantics # field.last-modified |
Browser compatibility
BCD tables only load in the browser
See also
Etag
- HTTP Conditional Requests guide
If-Match
,If-Modified-Since
,If-Unmodified-Since
,If-None-Match
conditional request headers304 Not Modified
,412 Precondition Failed
response status codes