If-Modified-Since - HTTP | MDN (original) (raw)

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The If-Modified-Since request HTTP header makes the request conditional: the server sends back the requested resource, with a200 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. UnlikeIf-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 associatedETag.

Syntax

If-Modified-Since: <day-name>, <day> <month> <year> <hour>:<minute>:<second> GMT

Directives

One of "Mon", "Tue", "Wed", "Thu", "Fri", "Sat", or "Sun" (case-sensitive).

2 digit day number, e.g. "04" or "23".

One of "Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec" (case sensitive).

4 digit year number, e.g. "1990" or "2016".

2 digit hour number, e.g. "09" or "23".

2 digit minute number, e.g. "04" or "59".

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

Browser compatibility

BCD tables only load in the browser

See also