Last-Modified header
Baseline
Widely available
This feature is well established and works across many devices and browser versions. It’s been available across browsers since July 2015.
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 request header | 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
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See also
Etag- HTTP Conditional Requests guide
If-Match,If-Modified-Since,If-Unmodified-Since,If-None-Matchconditional request headers304 Not Modified,412 Precondition Failedresponse status codes