The If-Modified-Since
request HTTP header makes the request conditional: the server will send back the requested resource, with a {{HTTPStatus("200")}} status, only if it has been last modified after the date given. If the request has not been modified since, the response will be a {{HTTPStatus("304")}} without any body; the {{HTTPHeader("Last-Modified")}} header will contain the date of last modification.
If used in combination with {{HTTPHeader("If-None-Match")}}, it is ignored, unless the server doesn't support If-None-Match
.
The most common use case is to update a cached entity that has no associated {{HTTPHeader("ETag")}}.
Header type | {{Glossary("Request header")}} |
---|---|
{{Glossary("Forbidden header name")}} | no |
Syntax
If-Modified-Since: <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
If-Modified-Since: Wed, 21 Oct 2015 07:28:00 GMT
Specifications
Specification | Title |
---|---|
{{RFC("7232", "Date", "3.3")}} | Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content |
Browser compatibility
The compatibility table in this page is generated from structured data. If you’d like to contribute to the data, please check out https://github.com/mdn/browser-compat-data/README.md and send us a pull request.
{{Compat}}
See also
- {{HTTPHeader("ETag")}}
- {{HTTPHeader("If-Unmodified-since")}}
- {{HTTPHeader("If-Match")}}
- {{HTTPHeader("If-None-Match")}}
- {{HTTPStatus("304")}}
Not Modified