how about
mgt1 ?E dtstart_z "2006-02-23T15:00:00"; # it starts at 1500Z?E dtstart_america_chicago "2006-02-23T15:00:00". # it starts at 1500 Chicago time
dtstart_america_chicago rdfs:subPropertyOf dtstart dtstart_z rdfs:subPropertyOf dtstart dtstart "2006-02-23T08:00:00"; # it starts at 3pm in some timezone -- Dan Connolly, W3C http://www.w3.org/People/Connolly/
