The Greenwich Meridian is a town called Greenwich longitude 0 degrees in the UK. GMT is the Foundation of all time zones in the world, the world time. Throughout the year, and does not switch to daylight saving time. All the other time zones to determine the local time to GMT. In New York, GMT standard time to about five hours, minus and minus 8 hours in California. So if GMT 1200 Pacific standard time, California will be 0400. Also known as Zulu time 1200 GMT GMT (noon), also for you can display weather charts, especially in the 12z/00Z 0000 GMT (midnight).