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 Amazon S3 returns the 304 Not Modified response code.

For more information about conditional requests, see RFC 7232.