5楼这个回复不严谨,会误导楼主.
HTTP Request-Header 中的 Date 这个字段并非是一定有的.并且可以说绝大部分时间不会包含这个字段
参考W3对于Header Field Definitions
14.18 Date
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Clients SHOULD only send a Date header field in messages that include an entity-body, as in the case of the PUT and POST requests, and even then it is optional. A client without a clock MUST NOT send a Date header field in a request.
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参考链接: 14 Header Field Definitionsto 楼主, 不使用JavaScript,无法保证获取到Client的时间.