Nowadays, Greenwich Mean Time, abbreviated as GMT, is a time zone designation rather than a time standard. Time difference between time zones can be expressed by the GMT or UTC offset. In the UTC standard, there is a commitment to keep within 0.9 seconds of GMT, so that every few years a leap second is applied to UTC. In the age of the internet, these small clock calibrations are conveniently distributed via NTP.
UTC/GMT-12 is 12 hours behind Greenwich Mean Time (GMT).
The GMT/UTC minus 12 hours offset is used for Baker Island and Howland Island, both uninhabited and located just north of the equator in the central Pacific Ocean about 3,100 kilometres (1,900 miles) southwest of Honolulu.
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The following countries or regions use the offset -12 either in Standard time or Daylight Saving Time as indicated.