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.

See full compatibility

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

http

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

http

If-Modified-Since: Wed, 21 Oct 2015 07:28:00 GMT

Specifications

Specification
HTTP Semantics # field.if-modified-since

Browser compatibility

	desktop					mobile
	Chrome	Edge	Firefox	Opera	Safari	Chrome Android	Firefox for Android	Opera Android	Safari on iOS	Samsung Internet	WebView Android	WebView on iOS
`If-Modified-Since`	Chrome – Full support Chrome 1 (Release date: 2008-12-11) footnote Full support	Edge – Full support Edge 12 (Release date: 2015-07-29) footnote Full support	Firefox – Full support Firefox 1 (Release date: 2004-11-09) footnote Full support	Opera – Full support Opera 15 (Release date: 2013-07-02) footnote Full support	Safari – Full support Safari 1 (Release date: 2003-06-23) footnote Full support	Chrome Android – Full support Chrome Android 18 (Release date: 2012-06-27) footnote Full support	Firefox for Android – Full support Firefox for Android 4 (Release date: 2011-03-29) footnote Full support	Opera Android – Full support Opera Android 14 (Release date: 2013-05-21) footnote Full support	Safari on iOS – Full support Safari on iOS 1 (Release date: 2007-06-29) footnote Full support	Samsung Internet – Full support Samsung Internet 1 (Release date: 2013-04-27) footnote Full support	WebView Android – Full support WebView Android 4.4 (Release date: 2013-12-09) footnote Full support	WebView on iOS – Full support WebView on iOS 1 (Release date: 2007-06-29) footnote Full support

Tip: you can click/tap on a cell for more information.

Full support: Full support