If-Modified-Since
The If-Modified-Since request HTTP header makes the
request conditional: the server sends back the requested resource, with a
200 status, only if it has been last modified after the given date. If
the resource has not been modified since, the response is a 304
without any body; the Last-Modified response header of a previous
request contains the date of 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 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 |
|---|
| Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests # header.if-modified-since |
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