I have an app that displays a timetable of certain ferry trips.
If I travel to a different timezone - say 4 hours behind, a 10am ferry trip now shows up as 6am?
You have seen NSDateFormatter's setTimeZone method, yes?
http://developer.apple.com/library/ios/documentation/Cocoa/Reference/Foundation/Classes/NSDateFormatter_Class/Reference/Reference.html#//apple_ref/occ/instm/NSDateFormatter/setTimeZone:
(b.t.w., I'd be amazed if there was a ferry that involved crossing four time zones; sounds like a cruise ship itinerary to me)