ifUnmodifiedSince

Return the object only if it has not been modified since the specified time; otherwise, return a 412 (precondition failed) error.

If both of the If-Match and If-Unmodified-Since headers are present in the request as follows:

• If-Match condition evaluates to true, and;

• If-Unmodified-Since condition evaluates to false;

Then Amazon S3 returns 200 OK and the data requested.

For more information about conditional requests, see RFC 7232.