{"json_modified": "2016-10-14T05:18:36.829743", "uuid": "c6c90f0f-aaa4-49e9-9576-e8eb641d43cb", "title": "If-Modified-Since", "url": "/en-US/docs/Web/HTTP/Headers/If-Modified-Since", "tags": ["HTTP Headers", "Reference", "Conditional Requests", "HTTP", "Request header"], "translations": [], "modified": "2016-10-14T05:18:36", "label": "If-Modified-Since", "localization_tags": [], "locale": "en-US", "id": 192697, "last_edit": "2016-08-31T04:26:37", "summary": "The <strong><code>If-Modified-Since</code></strong> request HTTP header makes the request conditional: the server will send back the requested resource, with a <a href=\"/en-US/docs/Web/HTTP/Status/200\" title=\"The HTTP 200 OK success status response code indicates that the request has succeeded. A 200 response is cacheable by default.\"><code>200</code></a> status, only if it has been last modified after the given date. If the request has not been modified since, the response will be a <a href=\"/en-US/docs/Web/HTTP/Status/304\" title=\"The HTTP 304 Not Modified client redirection response code indicates that there is no need to retransmit the requested resources. It is an implicit redirection to a cached resource. This happens when the request method is safe, like a GET or a HEAD request, or when the request is conditional and uses a If-None-Match or a If-Modified-Since header.\"><code>304</code></a> without any body; the <a href=\"/en-US/docs/Web/HTTP/Headers/Last-Modified\" title=\"The Last-Modified response HTTP header contains the date and time at which the origin server believes the resource was last modified. It is used as a validator to determine if a resource received or stored is the same. Less accurate than an ETag header, it is a fallback mechanism. Conditional requests containing If-Modified-Since or If-Unmodified-Since headers make use of this field.\"><code>Last-Modified</code></a> header will contain the date of last modification. Unlike <a href=\"/en-US/docs/Web/HTTP/Headers/If-Unmodified-Since\" title=\"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 request has been modified after the given date, the response will be a 412 (Precondition Failed) error.\"><code>If-Unmodified-Since</code></a>, <code>If-Modified-Since</code> can only be used with a <a href=\"/en-US/docs/Web/HTTP/Methods/GET\" title=\"The GET method requests a representation of the specified resource. Requests using GET should only retrieve data.\"><code>GET</code></a> or <a href=\"/en-US/docs/Web/HTTP/Methods/HEAD\" title=\"The HTTP HEAD method requests the headers that are returned if the specified resource would be requested with an HTTP GET method. Such a request can be done before deciding to download a large resource to save bandwidth, for example.\"><code>HEAD</code></a>.", "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-Modified-Since", "review_tags": []}