can be used to make a request method conditional by modification date: if the selected representation has been modified since the time specified in this field, then the server MUST NOT perform the requested operation and MUST instead respond with the 412 (Precondition Failed) status code.
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Description
can be used to make a request method conditional by modification date: if the selected representation has been modified since the time specified in this field, then the server MUST NOT perform the requested operation and MUST instead respond with the 412 (Precondition Failed) status code.
Syntax
If-Unmodified-Since: Mon, 17 Aug 2026 10:00:00 GMT
Related status codes
Related methods
Frequently asked questions
can be used to make a request method conditional by modification date: if the selected representation has been modified since the time specified in this field, then the server MUST NOT perform the requested operation and MUST instead respond with the 412 (Precondition Failed) status code.
It is classified as a request header and can appear in requests or responses depending on context.
Sources & standards
Updated 2026-09-13
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