blocks|key|5542577|text|根据the+spec，它用于年龄计算。如果您不知道服务器认为现在是什么时间，您将无法计算资源的“年龄”。以下是规范中的相关文本：|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|5542578|5542579|年龄计算算法摘要，当缓存收到响应时：|blockquote|5542580|5542581|age_value|style|BOLD|CODE|5542582|缓存通过此响应接收的Age:标头的值。|5542583|date_value|5542584|源站Date:头部的值|5542585|request_time|5542586|是缓存发出导致此缓存响应的请求的(本地)时间|5542587|response_time|5542588|是缓存接收响应的(本地)时间|5542589|now|5542590|是当前(本地)时间|5542591|apparent_age+=+max(0，response_time+-+date_value)；corrected_received_age+=+max(apparent_age，age_value)；response_delay+=+response_time+-+request_time；corrected_initial_age+=+corrected_received_age+%2B+response_delay；resident_time+=+now+-+response_time；current_age+=+corrected_initial_age+%2B+resident_time;|5542592|entityMap|0|LINK|mutability|MUTABLE|url|https://www.rfc-editor.org/rfc/rfc7231#section-7.1.1.2^0|2|8|0|0|0|0|0|0|9|0|9|0|0|0|A|0|A|0|2|5|0|0|C|0|C|0|0|0|D|0|D|0|0|0|3|0|3|0|0|0^^$0|@$1|2|3|4|5|6|7|1F|8|@]|9|@$A|1G|B|1H|1|1I]]|C|$]]|$1|D|3|-4|5|6|7|1J|8|@]|9|@]|C|$]]|$1|E|3|F|5|G|7|1K|8|@]|9|@]|C|$]]|$1|H|3|-4|5|6|7|1L|8|@]|9|@]|C|$]]|$1|I|3|J|5|6|7|1M|8|@$A|1N|B|1O|K|L]|$A|1P|B|1Q|K|M]]|9|@]|C|$]]|$1|N|3|O|5|6|7|1R|8|@]|9|@]|C|$]]|$1|P|3|Q|5|6|7|1S|8|@$A|1T|B|1U|K|L]|$A|1V|B|1W|K|M]]|9|@]|C|$]]|$1|R|3|S|5|6|7|1X|8|@$A|1Y|B|1Z|K|M]]|9|@]|C|$]]|$1|T|3|U|5|6|7|20|8|@$A|21|B|22|K|L]|$A|23|B|24|K|M]]|9|@]|C|$]]|$1|V|3|W|5|6|7|25|8|@]|9|@]|C|$]]|$1|X|3|Y|5|6|7|26|8|@$A|27|B|28|K|L]|$A|29|B|2A|K|M]]|9|@]|C|$]]|$1|Z|3|10|5|6|7|2B|8|@]|9|@]|C|$]]|$1|11|3|12|5|6|7|2C|8|@$A|2D|B|2E|K|L]|$A|2F|B|2G|K|M]]|9|@]|C|$]]|$1|13|3|14|5|6|7|2H|8|@]|9|@]|C|$]]|$1|15|3|16|5|6|7|2I|8|@]|9|@]|C|$]]|$1|17|3|-4|5|6|7|2J|8|@]|9|@]|C|$]]]|18|$19|$5|1A|1B|1C|C|$1D|1E]]]]

Per <a href="https://www.rfc-editor.org/rfc/rfc7231#section-7.1.1.2" rel="nofollow noreferrer">the spec</a>, it is used in age calculations. If you don't know what time the server thinks it is, you won't be able to calculate the &quot;age&quot; of a resource. Here's the relevant text from the spec:
<blockquote>
Summary of age calculation algorithm, when a cache receives a response:
<code>age_value</code> 
is the value of Age: header received by the cache with
this response.
<code>date_value</code> 
is the value of the origin server's <code>Date:</code> header
<code>request_time</code> 
is the (local) time when the cache made the request
that resulted in this cached response
<code>response_time</code> 
is the (local) time when the cache received the
response
<code>now</code> 
is the current (local) time
<pre><code>apparent_age = max(0, response_time - date_value);
corrected_received_age = max(apparent_age, age_value);
response_delay = response_time - request_time;
corrected_initial_age = corrected_received_age + response_delay;
resident_time = now - response_time;
current_age = corrected_initial_age + resident_time;
</code></pre>
</blockquote>

blocks|key|5542659|text|请考虑不使用Date标头，因为它在“禁止标头名称”的列表中。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|BOLD|entityRanges|data|5542660|中的以下描述可能会有所帮助：|5542661|5542662|禁止的标头名称是不能以编程方式修改的任何HTTP标头的名称；具体地说，是HTTP请求标头名称(与禁止的响应标头名称形成对比)。|5542663|禁止修改此类标头，因为用户代理保留对它们的完全控制。以Sec-开头的名称是为创建新的标题而保留的，这些标题不受使用Fetch的API的影响，这些API允许开发人员控制标题，如XMLHttpRequest。|5542664|禁止的标头名称以Proxy-或Sec-开头，或者是以下名称之一：|5542665|5542666|Accept-Charset|unordered-list-item|5542667|List+item|5542668|Accept-Encoding|5542669|Access-Control-Request-Headers|5542670|Access-Control-Request-Method|5542671|Connection|5542672|Content-Length|5542673|Cookie|5542674|Cookie2|5542675|Date|5542676|DNT|5542677|Expect|5542678|Host|5542679|Keep-Alive|5542680|Origin|5542681|Proxy-|5542682|Sec-|5542683|Referer|5542684|TE|5542685|Trailer|5542686|Transfer-Encoding|5542687|Upgrade|5542688|Via|5542689|5542690|entityMap^0|6|4|H|8|0|0|0|0|R|4|0|0|0|0|0|0|0|0|0|0|0|0|0|4|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|24|8|@$9|25|A|26|B|C]|$9|27|A|28|B|D]]|E|@]|F|$]]|$1|G|3|H|5|6|7|29|8|@]|E|@]|F|$]]|$1|I|3|-4|5|6|7|2A|8|@]|E|@]|F|$]]|$1|J|3|K|5|6|7|2B|8|@]|E|@]|F|$]]|$1|L|3|M|5|6|7|2C|8|@$9|2D|A|2E|B|C]]|E|@]|F|$]]|$1|N|3|O|5|6|7|2F|8|@]|E|@]|F|$]]|$1|P|3|-4|5|6|7|2G|8|@]|E|@]|F|$]]|$1|Q|3|R|5|S|7|2H|8|@]|E|@]|F|$]]|$1|T|3|U|5|S|7|2I|8|@]|E|@]|F|$]]|$1|V|3|W|5|S|7|2J|8|@]|E|@]|F|$]]|$1|X|3|Y|5|S|7|2K|8|@]|E|@]|F|$]]|$1|Z|3|10|5|S|7|2L|8|@]|E|@]|F|$]]|$1|11|3|12|5|S|7|2M|8|@]|E|@]|F|$]]|$1|13|3|14|5|S|7|2N|8|@]|E|@]|F|$]]|$1|15|3|16|5|S|7|2O|8|@]|E|@]|F|$]]|$1|17|3|18|5|S|7|2P|8|@]|E|@]|F|$]]|$1|19|3|1A|5|S|7|2Q|8|@$9|2R|A|2S|B|D]]|E|@]|F|$]]|$1|1B|3|1C|5|S|7|2T|8|@]|E|@]|F|$]]|$1|1D|3|1E|5|S|7|2U|8|@]|E|@]|F|$]]|$1|1F|3|1G|5|S|7|2V|8|@]|E|@]|F|$]]|$1|1H|3|1I|5|S|7|2W|8|@]|E|@]|F|$]]|$1|1J|3|1K|5|S|7|2X|8|@]|E|@]|F|$]]|$1|1L|3|1M|5|S|7|2Y|8|@]|E|@]|F|$]]|$1|1N|3|1O|5|S|7|2Z|8|@]|E|@]|F|$]]|$1|1P|3|1Q|5|S|7|30|8|@]|E|@]|F|$]]|$1|1R|3|1S|5|S|7|31|8|@]|E|@]|F|$]]|$1|1T|3|1U|5|S|7|32|8|@]|E|@]|F|$]]|$1|1V|3|1W|5|S|7|33|8|@]|E|@]|F|$]]|$1|1X|3|1Y|5|S|7|34|8|@]|E|@]|F|$]]|$1|1Z|3|20|5|S|7|35|8|@]|E|@]|F|$]]|$1|21|3|-4|5|6|7|36|8|@]|E|@]|F|$]]|$1|22|3|-4|5|6|7|37|8|@]|E|@]|F|$]]]|23|$]]

Please consider not to use the <code>Date</code> Header as it is on the list of the "Forbidden header names".

The following description from the <a href="https://developer.mozilla.org/en-US/docs/Glossary/Forbidden_header_name" rel="nofollow noreferrer">MDN web docs</a> might help:

<hr>

A forbidden header name is the name of any HTTP header that cannot be modified programmatically; specifically, an HTTP request header name (in contrast with a Forbidden response header name).

Modifying such headers is forbidden because the user agent retains full control over them. Names starting with <code>Sec-</code> are reserved for creating new headers safe from APIs using Fetch that grant developers control over headers, such as XMLHttpRequest.

Forbidden header names start with Proxy- or Sec-, or are one of the following names:

<ul>
<li>Accept-Charset</li>
<li>List item</li>
<li>Accept-Encoding</li>
<li>Access-Control-Request-Headers</li>
<li>Access-Control-Request-Method</li>
<li>Connection</li>
<li>Content-Length</li>
<li>Cookie</li>
<li>Cookie2</li>
<li>Date</li>
<li>DNT</li>
<li>Expect</li>
<li>Host</li>
<li>Keep-Alive</li>
<li>Origin</li>
<li>Proxy-</li>
<li>Sec-</li>
<li>Referer</li>
<li>TE</li>
<li>Trailer</li>
<li>Transfer-Encoding</li>
<li>Upgrade</li>
<li>Via</li>
</ul>

blocks|key|965311|text|只有为了更好地处理Expires报头，才需要Date：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|965312|Date:+Mon,+26+Mar+2012+12:53:02+GMT
Expires:+Wed,+25+Apr+2012+12:53:02+GMT|code-block|syntax|javascript|965313|服务器或客户端的时间可能不正确，因此客户端(浏览器)会尝试calculate+max+age+of+the+resource+freshness。这就是引入Cache-Control标签的原因之一。它使用秒数来过期，而不是固定时间。|965314|我测试了Chrome和Firefox，它们都是没有Date头的响应，所以可以安全地省略它，除非你仍然使用过时的Expires头。如果Date丢失，只需使用assumed+the+same+as+client's+time。在spec中，头是强制性的，这简直是疯了:日期格式化/解析消耗CPU和网络。|965315|entityMap|0|LINK|mutability|MUTABLE|url|https://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html#sec13.2.3|1|https://chromium.googlesource.com/chromium/src/%2B/refs/heads/master/net/http/http_response_headers.cc#1146^0|9|7|M|4|0|0|26|D|T|17|0|0|1J|7|1U|4|25|X|1|0^^$0|@$1|2|3|4|5|6|7|Y|8|@$9|Z|A|10|B|C]|$9|11|A|12|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|13|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|14|8|@$9|15|A|16|B|C]]|D|@$9|17|A|18|1|19]]|E|$]]|$1|M|3|N|5|6|7|1A|8|@$9|1B|A|1C|B|C]|$9|1D|A|1E|B|C]]|D|@$9|1F|A|1G|1|1H]]|E|$]]|$1|O|3|-4|5|6|7|1I|8|@]|D|@]|E|$]]]|P|$Q|$5|R|S|T|E|$U|V]]|W|$5|R|S|T|E|$U|X]]]]

The <code>Date</code> is needed only for a better work of <code>Expires</code> header:
<pre><code>Date: Mon, 26 Mar 2012 12:53:02 GMT
Expires: Wed, 25 Apr 2012 12:53:02 GMT
</code></pre>
A server or a client may have an incorrect time so client (browser) tries to <a href="https://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html#sec13.2.3" rel="nofollow noreferrer">calculate max age of the resource freshness</a>.
That was one of the reasons why the <code>Cache-Control</code> tag was introduced.
It uses seconds to expire instead of a fixed time.
I tested Chrome and Firefox and they are fine is response without Date header so it can be safely omitted unless you are still using obsolete <code>Expires</code> header. If the <code>Date</code> is missing it just <a href="https://chromium.googlesource.com/chromium/src/+/refs/heads/master/net/http/http_response_headers.cc#1146" rel="nofollow noreferrer">assumed the same as client's time</a>.
It's just insane that in spec the header is mandatory: the date formatting/parsing consumes CPU and network.

I have read <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.18" rel="noreferrer">RFC 2616</a>, but still I wonder, what the Date field is for. There is the Last-Modified field, that actually has a meaning besides just serving metadata, that is, for caching ('If-Modified-Since').

But what use has it to double the info in a separate Date header?

What's the rationale behind the HTTP Date header?

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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我已经阅读了，但我仍然想知道日期字段是用来做什么的。有一个Last-Modified字段，除了提供元数据外，它实际上还有一个含义，即用于缓存(‘If-Modified- field’)。但是，在单独的日期标题中将信息加倍又有什么用呢？

问HTTP Date报头背后的原理是什么？
EN

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问HTTP Date报头背后的原理是什么？EN