The Prime Meridian is the meridian of longitude that passes through Greenwich, England. On the opposite side of the Earth at 180 ° longitude lies an imaginary line called the International Dateline.

If you took a cruise on a ship circumventing, or traveling around the entire planet, you would experience firsthand a phenomena that would help you understand the importance of an international dateline. An explorer by the name of Ferdinand Magellan did just that in the year 1519. Magellan, along with 241 men, set out to travel around the entire Earth. These men kept very careful records as they traveled. Yet, when they returned, they found that their calendars were off by one day with the calendars of everyone in their home countries. What happened? How did these men loose an entire day? The day was not lost all at once. It was lost little by little, as they traveled around the Earth.
If you stay in one place, a day lasts 24 hours. However, if you travel the opposite direction of the rotation of the Earth, your day will be slightly longer than 24 hours, because you are traveling ahead of the setting sun. If you travel with the rotation of the Earth, your day will be slightly shorter than 24 hours because you are traveling into the sunset.

At any one moment there are actually two days on the Earth at the same time. A new day begins at midnight on the International Dateline. It travels around the Earth until 48 hours later it ends back at the International dateline. If you were to travel across the International Dateline, the date would change either forward or backward. This line actually lies in the middle of a time zone. This means that the hour on the clock would not change as you crossed over, just the date.