The time information which is included in digital signature can replace time stamp , therefore , it leaves out the uniform time stamp server that other PKI systems have to use .
同时由于签名中所含有的时间段信息，使每个证书都自动包含时间戳，省掉时间戳服务器，节省开销。
If the response file has the same Last-Modified time stamp, the server returns code 304 to the browser to tell it that the response file is the same.
如果响应文件拥有相同的Last - Modified时间戳，则服务器将返回代码304到浏览器，告知浏览器响应文件相同。
If the content is stale, the browser sends a request to the server to compare the Last-Modified time stamp.
如果内容失效，浏览器将发送一个请求到服务器以比较Last - Modified时间戳。
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