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If-Modified-Since

The If-Modified-Since request HTTP header makes the request conditional: the server will send back the requested resource, with a 200 status, only if it has been last modified after the given date. If the request has not been modified since, the response will be a 304 without any body; the Last-Modified header will contain 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 Title
RFC 7232, section 3.3: If-Modified-Since Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests

Browser compatibility

Feature Chrome Edge Firefox Internet Explorer Opera Safari Servo
If-Modified-Since(Yes)(Yes)(Yes)(Yes)(Yes)(Yes)(Yes)
Feature Android Chrome for Android Edge Mobile Firefox for Android IE Mobile Opera Mobile Safari Mobile
If-Modified-Since(Yes)(Yes)(Yes)(Yes)(Yes)(Yes)(Yes)

See also

Document Tags and Contributors

 Contributors to this page: fscholz, teoli
 Last updated by: fscholz,