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, S3 returns 200 OK and the data requested.

For more information about conditional requests, see RFC 7232.