North American Time Zones

Most professionals (astronomers, weather forecasters, pilots, etc) use a twenty four hour clock, referenced to Greenwich Mean Time (GMT), the time at Greenwich, England. The longitude there is 0 degrees; that's where the "Greenwich meridian" is. Greenwich Mean Time is also sometimes called Universal Time (UT). Note that GMT is always the same through the year (does not have a period of "summer" or "daylight savings" time).

To convert GMT to local time, or vice versa, we must know the time difference between GMT and local time. The time difference depends on the local longitude, and whether or not local time is standard or daylight savings time (one hour shifted). Not all states adopt daylight savings time in summer.

The Time Zones across North America and the Pacific from East to West are

            Standard   difference    Daylight   difference
              Time      from GMT    Savings    from GMT

Atlantic                -4 hours
Eastern       EST       -5 hours       EDT       -4 hours
Central       CST       -6 hours       CDT       -5 hours
Mountain      MST       -7 hours       MDT       -6 hours
Pacific       PST       -8 hours       PDT       -7 hours
Alaska                  -9 hours
Hawai                  -10 hours
Here is a link that gives longitude and latitude for major cities around the world.

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