Understanding time zones, specifically how to account for them, is an important part of travel planning, organising an international conference call, or simply to calculating the most convenient time to call a friend overseas. Time zones are 15 degrees of longitude each, so the world is split into 24 zones. The "default" time zone, Greenwich Mean Time, or GMT, is named for the Royal Observatory in Greenwich, England. It includes all points along the Prime Meridian, or 0 degrees longitude. All other time zones are defined as GMT plus or minus the number of hours they fall east or west of GMT.
Locate your starting point on a map and take note of its line of longitude. If you are in New York City, you will be 75 degrees west of the Prime Meridian, and thus five hours behind, or GMT -5. Los Angeles falls 120 degrees west of the Prime Meridian and is therefore at GMT -8. Asia, Australia, and most of Africa and Europe fall to the east of the Prime Meridian.
Locate the destination point in which you want to calculate the current time. Note its line of longitude. Again, note that points at least 15 degrees east of Greenwich will be at least one hour ahead of GMT, while points west of Greenwich will be behind GMT.
Count the number of lines of longitude between the two destinations. Always count from west to east; this will reduce confusion when you reach the International Date Line, which falls at the 180-degree line in the middle of the Pacific Ocean. The date to the west, or left on a map, of the International Date Line is always one day ahead of the date to the east of the line. The number of lines of longitude between your two destinations is the time difference between those two points in hours. If the two points are particularly far apart, for example, Los Angeles and Sydney, Australia, it may help to count from L.A. to the Prime Meridian and from the Prime Meridian to Sydney. Los Angeles' time zone is GMT -8 and Sydney's is GMT +10; thus, there is an 18-hour time difference between L.A. and Sydney.
Account for Daylight Saving Time and Summer Time. Most of North America observes Daylight Saving Time from the second Sunday in March until the second Sunday in November. In Europe, Summer Time is observed from the third Sunday in March until the fourth Sunday in October. In between these dates, every time zone will be thrown off by one hour. British Summer Time means that even the time along the Prime Meridian is changed to GMT +1. For the two weeks when DST is in effect in North America but Summer Time is not in effect in the UK, all time zones in North America are one hour closer to GMT. For example, New York is GMT -4 for two weeks in March and again for two weeks in November. You will need to check if your destination country observes Daylight Saving Time or Summer Time to get an accurate time difference calculation.