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GMT Explained


Many of the graphics and tables on this site use times labeled GMT, UTC, or Z. You've probably noticed that none of these times seem to match your local time.

To avoid the confusion caused by all the time zones around the world and changes between standard time and daylight savings time, most scientists use the twenty four hour clock and only one time zone, Greenwich Mean Time or GMT.

GMT time is the current time measured on the Prime Meridian (0 degrees longitude). The Prime Meridian runs through Greenwich, England, hence the name Greenwich Mean Time or GMT.

Just to make matters a little more confusing, other people have decided to use the names Z or Zulu and UTC or Universal Time Coordinates instead of GMT. GMT, Z, and UTC all mean the same thing, the current time at the Prime Meridian.

In the US, you can convert from GMT, Z, or UTC to your local time by using the following table:

                             Subtract this many hours
If your time zone is:        to get your current time          Formula
Eastern Daylight Time (EDT)             4                 GMT - 4 hours = EDT
Eastern Standard Time (EST)             5                 GMT - 5 hours = EST
Central Daylight Time (CDT)             5                 GMT - 5 hours = CDT
Central Standard Time (CST)             6                 GMT - 6 hours = CST
Mountain Daylight Time (MDT)            6                 GMT - 6 hours = MDT
Mountain Standard Time (MST)            7                 GMT - 7 hours = MST
Pacific Daylight Time (PDT)             7                 GMT - 7 hours = PDT
Pacific Standard Time (PST)             8                 GMT - 8 hours = PST