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 resource has been modified after the given date, the response will be a
412 (Precondition Failed) error.
There are two common use cases:
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.
If-Rangeheader, it can be used to ensure that the new fragment requested comes from an unmodified document.
If-Unmodified-Since: <day-name>, <day> <month> <year> <hour>:<minute>:<second> GMT
Greenwich Mean Time. HTTP dates are always expressed in GMT, never in local time.
If-Unmodified-Since: Wed, 21 Oct 2015 07:28:00 GMT
|RFC 7232, section 3.4: If-Unmodified-Since||Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests|
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