Chamorro Time Zone

The Chamorro Time Zone, formerly the Guam Time Zone, is a United States time zone which observes standard time ten hours ahead of Coordinated Universal Time (UTC+10:00). The clock time in this zone is based on the mean solar time of the 150th meridian east of the Greenwich Observatory.

The zone includes the U.S. territories of Guam and the Northern Mariana Islands, where the Chamorro people are the original inhabitants. Since Daylight Saving Time (DST) is not observed anywhere in this zone, the time is always known as Chamorro Standard Time (ChST).

The zone is two hours behind Wake Island Time Zone and 15 hours ahead of North American Eastern Time Zone. As of March 19, 2024 it is the easternmost (farthest ahead) time zone shown on the U.S. government's time.gov webpage. Although Wake, Howland and Baker Islands are U.S. territories in more easterly time zones (that are not shown), they are uninhabited.

Chamorro Standard Time shares the same time as Australian Eastern Standard Time. The State of Queensland also does not observe DST, so the Commonwealth of the Northern Mariana Islands (CNMI), Guam and Queensland are on the same time year round.

Populated areas and major cities

 * Hagåtña, Guam
 * Saipan, Northern Mariana Islands

History
Until the end of 1844, Guam and Mariana Islands belonged to Captaincy General of the Philippines, which followed the date of the western hemisphere on their island, placing them on the eastern side of the International Date Line (IDL). At the end of Monday, December 30, 1844, both Guam and Mariana Islands continued directly to Wednesday, January 1, 1845, effectively removing Tuesday, December 31, 1844 from the calendar to follow the eastern hemisphere on the west side of the IDL and realign themselves from American to Asian dates.

Before time zones were introduced, every place used local observation of the sun to set its clocks, which meant that every location used a different local mean time based on its longitude. For example, Hagåtña, the capital of Guam at the time, at longitude 144°45′E, had a local time equivalent to GMT-14:21 under the date of the western hemisphere and GMT+09:39 under the eastern hemisphere.

In 1901, "Guam Standard Time" was established within the United States as GMT+10:00.

The U.S. Congress formally created Chamorro Standard Time by federal Public Law 106-564 on December 23, 2000. Previously, Guam Standard Time existed per territorial Public Law 5-25 (1959). This statute is still on the territorial law books, although it is preempted by the federal law.

The Northern Mariana Islands, as part of the Trust Territory of the Pacific Islands, were not part of any American time zone prior to becoming a U.S. Commonwealth on January 1, 1978. Being directly north of Guam, however, they observed the same time.