{"json_modified": "2016-10-14T05:19:12.940800", "uuid": "0e86f26c-d233-4623-b199-b0010c406eec", "title": "Last-Modified", "url": "/en-US/docs/Web/HTTP/Headers/Last-Modified", "tags": ["HTTP Headers", "Response Header", "Reference", "HTTP"], "translations": [], "modified": "2016-10-14T05:19:13", "label": "Last-Modified", "localization_tags": [], "locale": "en-US", "id": 192723, "last_edit": "2016-10-11T06:47:13", "summary": "The <strong><code>Last-Modified</code></strong> 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 <a href=\"/en-US/docs/Web/HTTP/Headers/ETag\" title=\"The ETag HTTP response header is an identifier for a specific version of a resource. It allows caches to be more efficient, and saves bandwidth, as a web server does not need to send a full response if the content has not changed. On the other side, if the content has changed, etags are useful to help prevent simultaneous updates of a resource from overwriting each other (&quot;mid-air collisions&quot;).\"><code>ETag</code></a> header, it is a fallback mechanism. Conditional requests containing <a href=\"/en-US/docs/Web/HTTP/Headers/If-Modified-Since\" title=\"The If-Modified-Since request HTTP header makes the request conditional: the server will send back the requested resource, with a 200 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 304 without any body; the Last-Modified header will contain the date of last modification. Unlike If-Unmodified-Since, If-Modified-Since can only be used with a GET or HEAD.\"><code>If-Modified-Since</code></a> or <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> headers make use of this field.", "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/Last-Modified", "review_tags": []}