How to calculate time zone changes

Written by joshua smyth
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How to calculate time zone changes
Time zones are North-South bands that keep the same time. (black magnifier on world map with time zones image by Anatoly Tiplyashin from

In a world where international travel and communication are routine, knowing how to quickly calculate how time changes across time zones becomes an essential management skill. A manager in Ohio might be responsible for a teamed based in Paris, New Delhi and Tokyo, and needs to know when his team is reachable, for example. When you are travelling and want to call home, knowing how to calculate time zone changes will help you avoid startling your family in the middle of the night. Calculating these changes is easy with a time zone map.

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Things you need

  • Time zone map
  • Clock set to local time

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  1. 1

    Buy a time zone map or look one up online. Although the world is theoretically broken up into 24 time zones that are each 15 degrees of longitude wide and one hour apart, these even bands really only apply in the ocean. On land, time zones often follow national or regional borders, and looking at a map is the only way to check.

  2. 2

    Find your current location on the map and note what time zone it is in. Time zones are written relative to Greenwich Mean Time (GMT), which is the local time at the Royal Observatory in Greenwich, England. Time zones are written as GMT+x or GMT-x, where "x" is the number of hours by which the time differs. Since the Earth turns from west to east, the time gets later as you move east and earlier as you move west. This means that the 12 time zones to the east of Greenwich are marked GMT+1 to GMT+12, while the other time zones are marked from GMT-1 to GMT-12. The line where GMT+12 and GMT-12 meet is the international dateline. Crossing it going east brings you into the next day, while going west brings you to the day before. Many time zones now refer to UTC (coordinated universal time) instead, which is the same as GMT but ignores daylight savings.

  3. 3

    Find the location for which you wish to calculate the time zone change n the map and note its time zone number.

  4. 4

    Add the differences between UTC/GMT of your home time zone and the other zone. For example, the U.S. East Coast is at UTC-5, while Moscow is UTC+3. Adding 5 and 3 makes 8, which is the difference in hours between the two times. You can also simply count the number of zones between the two places on the map, as each one is a one-hour difference (although many countries use half-hour increments within the zone as well).

  5. 5

    Check for daylight savings time. Some countries move forward or back an hour to adjust for seasonal variations in sunlight. This is more likely in the far north or south. Many online time zone calculators also list countries that use daylight savings time.

  1. 1

    Use a map with grid lines to calculate the degrees of longitude between two places.

  2. 2

    Divide the number you get by 15. The result is the number of one-hour time zones between the two.

  3. 3

    Check your figure against a map to be sure. Time zone lines deviate widely from the ideal 15-degree increments. These provide an exact figure for local time, but do not take into account the political decision of which city's clocks to follow.

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