  You need to take some care when planning trips that span time zones, e.g.,:  Your "body clock" may experience some stress as you "tell" it to meet business appointments, tours and other obligations perhaps a few or several hours different from the hours you normally rest.  You may miss important obligations simply by not understanding what will be the correct local time as you travel.  Crossing the International Date Line can cause confusion about on what date you'll arrive, e.g.,:  Starting a 00-00 hour flight from the U.S. west coast to Japan or Hong Kong in late evening can land you there in the morning 