>From http://www.w3.org/Protocols/rfc2616/rfc2616-sec9.html

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The semantics of the GET method change to a "conditional GET" if the request message includes an If-Modified-Since, If-Unmodified-Since, If-Match, If-None-Match, or If-Range header field. A conditional GET method requests that the entity be transferred only under the circumstances described by the conditional header field(s). The conditional GET method is intended to reduce unnecessary network usage by allowing cached entities to be refreshed without requiring multiple requests or transferring data already held by the client.
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>From Nginx Wiki (directive in HTTP Core):



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if_modified_since

syntax: if_modified_since [off|exact|before]

default: if_modified_since exact

context: http, server, location

The directive (0.7.24) defines how to compare time of file modification and time in request header "If-Modified-Since":


off