If-Modified-Since

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 header name	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

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