If-Unmodified-Since - HTTP 编辑

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 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-Range header, it can be used to ensure that the new fragment requested comes from an unmodified document.

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

Browser compatibility

BCD tables only load in the browser

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.

See also

• Last-Modified
• If-Modified-Since
• If-Match
• If-None-Match
• If-Range
• 412Precondition Failed