tried with 6.2.0.ER5 Last-Modified header always contains "Thu, 01 Jan 1970 00:00:00 GMT" see https://issues.jboss.org/browse/ISPN-3204
Michal Linhard <mlinhard> made a comment on jira ISPN-3204 I can see the reason why Last-Modified value is not maintained, but in case we don't wanna implement it we should at least clearly document that.