ifModifiedSince

Return the object only if it has been modified since the specified time; otherwise, return a 304 Not Modified error.

If both of the If-None-Match and If-Modified-Since headers are present in the request as follows:If-None-Match condition evaluates to false, and; If-Modified-Since condition evaluates to true; then, S3 returns 304 Not Modified status code.

For more information about conditional requests, see RFC 7232.