单项选择题

Strange things happen to time when you travel, because the earth is divided into twenty-four time zones, and one hour separate. You can have days with more or fewer than twenty-four hours, and weeks with more or fewer than seven days.
If you make a five-day trip across the Atlantic Ocean, your ship enters a different time zone every day. As you enter each zone, the time changes one hour. Traveling west, you set your clock back; traveling east, you set it ahead. Each day of your trip has either twenty-five or twenty-three hours.
If you travel by ship across the Pacific, yon cross the international date line. By agreement, this is the point where a new day begins. When you cross the line, you change your calendar one full day, backward or forward. Traveling east, today becomes yesterday; traveling west, it is tomorrow.
When you cross the Atlantic Ocean traveling west, you should set clock ______.