请教一道关于时差的问题

Pumpkin

题目：
A train travels from New York City to Chicago, a distance of approximately 840 miles, at an average rate of 60 miles per hour and arrrives in Chicago at 6:00 in the evening, Chicago time. At whate hour in the morning, New York City time, did the train depart for Chicago? (Chicago time is one hour EARLIER than New York City time.)
答案：5:00
我有些疑惑，总共行程时间是840/60=14小时，Chicago 比 New York City 早1个小时，就是说到达Chicago时，New York City是下午5:00(17:00), 也就是说应该是早上 17:00-14=3:00点出发的，为什么是5:00呢？难道earlier的意思是加一个小时吗？这不太可能吧。