If-Modified-Since 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 If-Modified-Since request header makes a request conditional.
The server sends back the requested resource, with a 200 status, only if it has been modified after the date in the If-Modified-Since header.
If the resource has not been modified since, the response is a 304 without any body, and the Last-Modified response header of the previous request contains the date of the last modification.
Unlike If-Unmodified-Since, If-Modified-Since can only be used with a GET or HEAD.
When used in combination with 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 ETag.
| Header type | Request header |
|---|---|
| Forbidden request header | 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, orSun(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 |
|---|
| HTTP Semantics # field.if-modified-since |
Browser compatibility
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See also
ETagIf-Match,If-None-Match,If-Unmodified-Sinceconditional request headers304 Not Modified,412 Precondition Failedresponse status codes