The HTTPIf-Modified-Sincerequest header makes a requestconditional.The server sends back the requested resource, with a200 status, only if it has been modified after the date in theIf-Modified-Since header.If the resource has not been modified since, the response is a304 without any body, and theLast-Modified response header of the previous request contains the date of the last modification.

UnlikeIf-Unmodified-Since,If-Modified-Since can only be used with aGET orHEAD.When used in combination withIf-None-Match, it is ignored, unless the server doesn't supportIf-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

<day-name>

One ofMon,Tue,Wed,Thu,Fri,Sat, orSun (case-sensitive).

<day>

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

<month>

One ofJan,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

Browser compatibility

See also

• ETag
• If-Match,If-None-Match,If-Unmodified-Since conditional request headers
• 304 Not Modified,412 Precondition Failed response status codes