为了研究睡眠究竟怎样影响员工工资，研究人进行了一项自然试验——利用美国时区日落时间。
To investigate how sleep affects worker wages, the researchers took advantage of a kind of natural experiment - sunset times across American time zones.
如果你在美国的其他地方跟他人进行电话会谈安排，要明确你考虑的时区。
If you're arranging a conference call with people in other parts of the United States, be clear on the time zone you have in mind.
美国直到1918年才把时区写入法律。
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