Time Zone Abbreviations Explained (EST, EDT, CST, CDT, MST, MDT, PST, PDT and More)

EST/EDT: Eastern Standard/Daylight Time (UTC-5/-4). CST/CDT: Central Standard/Daylight Time (UTC-6/-5). MST/MDT: Mountain Standard/Daylight Time (UTC-7/-6). PST/PDT: Pacific Standard/Daylight Time (UTC-8/-7).

The 'S' abbreviation applies in winter (standard time); the 'D' abbreviation applies in summer (daylight time) — except in zones that don't observe daylight saving at all, like Arizona's Mountain Standard Time, which stays MST year-round.

AKST/AKDT: Alaska Standard/Daylight Time (UTC-9/-8). HST: Hawaii Standard Time (UTC-10, no daylight saving in Hawaii itself); HDT applies only in the Aleutian Islands, which do observe daylight saving even though they share the same zone name as Hawaii.

AST: Atlantic Standard Time (UTC-4, Puerto Rico and the US Virgin Islands, no daylight saving). SST: Samoa Standard Time (UTC-11, American Samoa, no daylight saving). ChST: Chamorro Standard Time (UTC+10, Guam and the Northern Mariana Islands, no daylight saving) — the only US abbreviation for a zone ahead of UTC rather than behind it.

Using the full abbreviation rather than just a zone name avoids ambiguity in writing — '3 PM CST' is unambiguous, while '3 PM Central' during the summer months technically ought to be labeled CDT instead, since the zone spends roughly eight months of the year in daylight saving.

This matters most in written schedules, invoices, and shipping manifests that get read long after they were created, when there's no calendar context to infer whether standard or daylight time was intended.

For anyone unsure which abbreviation to use, pairing it with the explicit UTC offset — 'CDT (UTC-5)' rather than just 'CDT' — removes any remaining ambiguity, especially useful when writing for an international audience unfamiliar with US zone naming conventions.