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If-Unmodified-Since

The If-Unmodified-Since request HTTP header makes the request conditional: the server will send back the requested resource, or accept it in the case of a POST or another non-safe method, only if it has not been last modified after the given date. If the request has been modified after the given date, the response will be a 412 (Precondition Failed) error.

There are two common use cases:

  • In conjunction non-safe methods, like POST, it can be used to implement an optimistic concurrency control, like done by some wikis: editions are rejected if the stored document has been modified since the original has been retrieved.
  • In conjunction with a range request with a If-Range header, it can be used to ensure that the new fragment requested comes from an unmodified document.
Header type Request header
Forbidden header name no

Syntax

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

Specifications

Specification Title
RFC 7232, section 3.4: If-Unmodified-Since Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests

Browser compatibility

Feature Chrome Edge Firefox Internet Explorer Opera Safari Servo
If-Unmodified-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-Unmodified-Since(Yes)(Yes)(Yes)(Yes)(Yes)(Yes)(Yes)

See also

Document Tags and Contributors

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