If-Unmodified-Since

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

The If-Unmodified-Since header is commonly used in the following situations:

In conjunction with non-safe methods like POST, this header can be used to implement an optimistic concurrency control, as is done by some wikis: revision are rejected if the stored document has been modified since the original was retrieved, avoiding conflicts.
In conjunction with a range request using the Range header, this header can be used to ensure that the new fragment requested comes from an unmodified document.

Header type	Request header
Forbidden header name	No

Syntax

http

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

http

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

Specifications

Specification
HTTP Semantics # field.if-unmodified-since

Browser compatibility

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