As the javadoc for @CreationTimestamp and @UpdateTimestamp state they explicitly work with "in VM" generation". See als the documentation for details : http://docs.jboss.org/hibernate/orm/5.1/topical/html_single/generated/GeneratedValues.html

FWIW I have also created a Jira to move the discussion at the link above to the User Guide : https://hibernate.atlassian.net/browse/HHH-10573