I'd be interested in seeing the work you did to get your answer, just look at the times, if one is 20 minutes after, the other is 40 minutes after, and the time zone shift is in even hour increments, how would it be possible to get 17 hours even? I think perhaps the way you're calculating the answer is actually 100% correct, but you're forgetting to carry that extra 20 minutes somewhere in your work.