Hawaii–Aleutian Time Zone

Time zone covering Hawaii and western Aleutian Islands (UTC−10 standard). Hawaii stays on HST year‑round; the Aleutians may observe daylight saving time and shift to UTC−9 (HDT). Includes parts of French Polynesia.

The Hawaii–Aleutian Time Zone is a time offset region that uses UTC−10 during standard time and UTC−9 during daylight saving time where observed. In U.S. practice the zone is called Hawaii–Aleutian Standard Time (HST) for the standard offset and Hawaii–Aleutian Daylight Time (HDT) when the Aleutian portion observes daylight saving. The reference for the offset is Coordinated Universal Time.

Offset and naming

During standard time the zone is UTC−10. Portions of the zone that use daylight saving move clocks forward one hour to UTC−9 for daylight saving time. In descriptive usage the season names used are the same as in other U.S. time designations: winter for standard time and summer for daylight time, often written as HST in winter and HDT in summer where applicable.

Geographic extent and jurisdictions

The time zone primarily serves parts of the United States and several Pacific island groups. Key areas are:

Hawaii — the entire state observes HST year‑round and does not observe daylight saving time.
Parts of Alaska — specifically the western Aleutian Islands located west of 169.5° W longitude. Those Aleutian islands follow the Hawaii–Aleutian offset; they typically observe U.S. daylight saving rules and switch to HDT in summer.
Several islands in the central Pacific use the same UTC−10 offset (though not always under the same name), notably parts of French Polynesia such as Tahiti and the Society Islands.

History and practice

The arrangement reflects both geographic longitude and political timekeeping choices. The legal boundary in Alaska that separates the Aleutian portion in this zone from the rest of Alaska is defined by longitude. Hawaii opted long ago to remain on standard time year‑round; by contrast the Aleutian portion follows U.S. daylight saving conventions, producing a seasonal one‑hour difference between Hawaii and those Aleutian islands.

Practical implications and notable facts

Because Hawaii does not change clocks, local time relationships with North American and other Pacific locations shift when others enter or leave daylight saving time. For example, in winter Hawaii often shares the same clock time as the western Aleutians; in summer the Aleutians advance one hour ahead. Many Pacific territories use UTC−10 without adopting the "Hawaii–Aleutian" label, so it is common to encounter differing local names for the same numerical offset.

The Hawaii–Aleutian Time Zone is used for civil, commercial and transportation scheduling across the covered islands and is important for aviation, shipping and communications in the central and eastern Pacific.