UTC−12:00 is an identifier for a time offset from UTC of −12 hours.

As standard time (all year round)

• Baker Island
  Baker Island
  Baker Island is an uninhabited atoll located just north of the equator in the central Pacific Ocean about southwest of Honolulu. The island lies almost halfway between Hawaii and Australia, and is a possession of the United States. Its nearest neighbor is Howland Island, to the north.Located at...
  
   and Howland Island
  Howland Island
  Howland Island is an uninhabited coral island located just north of the equator in the central Pacific Ocean, about southwest of Honolulu. The island lies almost halfway between Hawaii and Australia and is an unincorporated, unorganized territory of the United States. Geographically, it is part...
  
   (uninhabited islands belonging to the United States) are assigned to this time zone.

UTC−12 is a nautical time zone comprising the high seas between 180°

180th meridian

The 180th meridian or antimeridian is the meridian which is 180° east or west of the Prime Meridian passing through the Royal Observatory, Greenwich. It is common to both east longitude and west longitude. It is used as the basis for the International Date Line because it for the most part passes...

 and 172°30′W longitude. Subtract twelve hours from Coordinated Universal Time

Coordinated Universal Time

Coordinated Universal Time is the primary time standard by which the world regulates clocks and time. It is one of several closely related successors to Greenwich Mean Time. Computer servers, online services and other entities that rely on having a universally accepted time use UTC for that purpose...

to obtain the time for this zone. Ships using this time are the last to begin each calendar day. No human habitations are in this time zone.