If-Modified-Since

参考： If-Modified-Since

14.25 If-Modified-Since

The If-Modified-Sincerequest-header field is used with a method to make it conditional: ifthe requested variant has not been modified since the time specified inthis field, an entity will not be returned from the server; instead, a304 (not modified) response will be returned without any message-body.

       If-Modified-Since = "If-Modified-Since" ":" HTTP-date

An example of the field is:

       If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT

A GET method with an If-Modified-Sinceheader and no Range header requests that the identified entity betransferred only if it has been modified since the date given by the If-Modified-Since header. The algorithm for determining this includes the following cases:

      a) If the request would normally result in anything other than a         200 (OK) status, or if the passed If-Modified-Since date is         invalid, the response is exactly the same as for a normal GET.         A date which is later than the server's current time is         invalid.

      b) If the variant has been modified since the If-Modified-Since         date, the response is exactly the same as for a normal GET.

      c) If the variant has not been modified since a valid If-         Modified-Since date, the server SHOULD return a 304 (Not         Modified) response.

The purpose of this feature is to allow efficient updates of cached information with a minimum amount of transaction overhead.

      Note: The Range request-header field modifies the meaning of If-      Modified-Since; see section 14.35 for full details.

      Note: If-Modified-Since times are interpreted by the server, whose      clock might not be synchronized with the client.

      Note: When handling an If-Modified-Since header field, some      servers will use an exact date comparison function, rather than a      less-than function, for deciding whether to send a 304 (Not      Modified) response. To get best results when sending an If-      Modified-Since header field for cache validation, clients are      advised to use the exact date string received in a previous Last-      Modified header field whenever possible.

      Note: If a client uses an arbitrary date in the If-Modified-Since      header instead of a date taken from the Last-Modified header for      the same request, the client should be aware of the fact that this      date is interpreted in the server's understanding of time. The      client should consider unsynchronized clocks and rounding problems      due to the different encodings of time between the client and      server. This includes the possibility of race conditions if the      document has changed between the time it was first requested and      the If-Modified-Since date of a subsequent request, and the

      possibility of clock-skew-related problems if the If-Modified-      Since date is derived from the client's clock without correction      to the server's clock. Corrections for different time bases      between client and server are at best approximate due to network      latency.

The result of a request having both an If-Modified-Since header field and either an If-Match or an If-Unmodified-Since header fields is undefined by this specification. 

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什么函数库可以把HTTP应答的“Last-Modified: Wed, 03 Sep 2008 05:12:12 GMT”这样的日期格式转化为COleDateTime或其他数据格式也可以。