How come my computer has two different GMTs?

Question

On my time and date control, my computer has two different GMT timezones. One at Casablanca and one called Greenwich Mean Time. The trouble is that the latter is one hour ahead of the former. Which one do people generally use? People say it's the Greenwich Mean Time but when I use a time zone converter online, it seems to agree with the Casablanca one(the one that is one hour later). I'm really confused. I live in the Pacific Timezone which is GMT-8 so it should be Greenwich but the converted time I got from the converters all give me time that's fit for GMT-7, not -8.

I need to check something tomorrow at 12:30GMT. Shouldn't that be 4:30 where I live? The converters say it's 5:30.

Answer 1

Historical Note

Greenwich Mean Time is a widely used historical term, but one that has been used in several ways. Because of the ambiguity, its use is no longer recommended in technical contexts.

Prior to 1925, in astronomical and nautical almanacs, a day of Greenwich Mean Time began at noon. This reckoning of Greenwich Mean Time is now called Greenwich Mean Astronomical Time, and is no longer used. Persons using old editions of the almanacs for historical research should be aware of the previous convention.

For more information on time, time scales, and accurate clocks, see the U.S. Naval Observatory Time Service Department web pages. Related information can be found on the pages of the National Institute of Standards and Technology (NIST).

http://physics.nist.gov/GenInt/Time/time...

The U.S. Naval Observatory (USNO) is charged with the responsibility for precise time determination and management of time dissemination . Modern electronic systems, such as electronic navigation or communications systems, depend increasingly on precise time and time interval (PTTI). Examples would be the ground-based LORAN-C navigation system and the satellite-based Global Positioning System (GPS). These systems are based on the travel time of the electromagnetic signals: an accuracy of 10 nanoseconds (10 one-billionths of a second) corresponds to a position accuracy of 10 feet. In fast communications, time synchronization is equally important. All of these official systems are referenced to the USNO Master Clock.

Julian Day Number is a count of days elapsed since Greenwich mean noon on 1 January 4713 B.C., Julian proleptic calendar. The Julian Date is the Julian day number followed by the fraction of the day elapsed since the preceding noon.

http://www.timeanddate.com/worldclock/

Answer 2

It's down to Daylight Saving Time (DST). During the summer, Britain, for example, which uses GMT normally, switches to BST, which is 1 hour ahead. Other GMT areas do not switch at the same time, and so on a computer they will be listed as a separate GMT entry so that local users always have the correct time. So while you may select GMT, the time will not actually be GMT because that area is not using GMT at the time.

For future reference, GMT will always be the earlier of the times available.