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:
- In conjunction with 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-Rangeheader, it can be used to ensure that the new fragment requested comes from an unmodified document.
|Header type||Request header|
|Forbidden header name||no|
If-Unmodified-Since: <day-name>, <day> <month> <year> <hour>:<minute>:<second> GMT
- One of "Mon", "Tue", "Wed", "Thu", "Fri", "Sat", or "Sun" (case-sensitive).
- 2 digit day number, e.g. "04" or "23".
- One of "Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec" (case sensitive).
- 4 digit year number, e.g. "1990" or "2016".
- 2 digit hour number, e.g. "09" or "23".
- 2 digit minute number, e.g. "04" or "59".
- 2 digit second number, e.g. "04" or "59".
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|
The compatibility table in this page is generated from structured data. If you'd like to contribute to the data, please check out https://github.com/mdn/browser-compat-data and send us a pull request.
|Chrome Full support Yes||Edge Full support Yes||Firefox Full support Yes||IE Full support Yes||Opera Full support Yes||Safari Full support Yes||WebView Android Full support Yes||Chrome Android Full support Yes||Edge Mobile Full support Yes||Firefox Android Full support Yes||Opera Android Full support Yes||Safari iOS Full support Yes||Samsung Internet Android Full support Yes|
- Full support
- Full support