{"json_modified": "2016-10-14T05:19:12.595802", "uuid": "8463078d-750f-4111-bb8e-4e524e4a3b8e", "title": "If-Unmodified-Since", "url": "/en-US/docs/Web/HTTP/Headers/If-Unmodified-Since", "tags": ["HTTP Headers", "Reference", "HTTP", "Request header"], "translations": [], "modified": "2016-10-14T05:19:12", "label": "If-Unmodified-Since", "localization_tags": [], "locale": "en-US", "id": 192701, "last_edit": "2016-08-31T04:27:18", "summary": "The <strong><code>If-Unmodified-Since</code></strong> request HTTP header makes the request conditional: the server will send back the requested resource, or accept it in the case of a <a href=\"/en-US/docs/Web/HTTP/Methods/POST\" title=\"The HTTP POST method sends data to the server. The type of the body of the request is indicated by the Content-Type header.\"><code>POST</code></a> or another non-<a class=\"glossaryLink\" href=\"/en-US/docs/Glossary/safe\" title=\"safe: An HTTP method is safe if it doesn't alter the state of the server. In other words, a method is safe if it leads to a read-only operation. Several common HTTP methods are safe: GET, HEAD, or OPTIONS. All safe methods are also idempotent as well as some, but not all, unsafe methods like PUT, or DELETE.\">safe</a> method, only if it has not been last modified after the given date. If the request has been modified after the given date, the response will be a <a href=\"/en-US/docs/Web/HTTP/Status/412\" title=\"The HTTP 412 Precondition Failed client error response code indicates that access to the target resource has been denied. This happens with conditional requests on methods other than GET or HEAD when the condition defined by the If-Unmodified-Since or If-None-Match headers is not fulfilled. In that case, the request, usually an upload or a modification of a resource, cannot be made and this error response is sent back.\"><code>412</code></a> (Precondition Failed) error.", "sections": [{"id": "Quick_Links", "title": null}, {"id": "Syntax", "title": "Syntax"}, {"id": "Directives", "title": "Directives"}, {"id": "Examples", "title": "Examples"}, {"id": "Specifications", "title": "Specifications"}, {"id": "Browser_compatibility", "title": "Browser compatibility"}, {"id": "See_also", "title": "See also"}], "slug": "Web/HTTP/Headers/If-Unmodified-Since", "review_tags": []}