US Time Zone Abbreviations Explained

Last reviewed on May 11, 2026

If a colleague writes "let's meet at 3 EST in June" or a flight is listed in PDT in January, something is off. US time zone abbreviations encode whether a zone is currently observing daylight saving time, and using the wrong one can shift a meeting by a full hour. This page walks through every abbreviation, when to use which, and the practical rule that avoids almost all of the confusion.

The structure of US zone abbreviations

Almost every US time zone abbreviation is three letters and follows the same pattern:

1. First letter — the zone (E, C, M, P, AK, H).
2. Middle letter — S for "Standard" or D for "Daylight".
3. Last letter — T for "Time".

So EST = Eastern Standard Time, EDT = Eastern Daylight Time. Two-letter versions like ET (Eastern Time) and CT (Central Time) drop the middle letter and refer to the zone in general, regardless of whether DST is in effect.

Full table of US abbreviations

Zone	General	Standard (winter)	Daylight (summer)	UTC offset (std / dst)
Eastern	ET	EST	EDT	UTC−5 / UTC−4
Central	CT	CST	CDT	UTC−6 / UTC−5
Mountain	MT	MST	MDT	UTC−7 / UTC−6
Pacific	PT	PST	PDT	UTC−8 / UTC−7
Alaska	AKT	AKST	AKDT	UTC−9 / UTC−8
Hawaii-Aleutian	HAT	HST	HDT	UTC−10 / UTC−9

A few notes that affect the bottom rows:

• Hawaii observes HST year-round. Because Hawaii does not observe daylight saving time, you will rarely see HDT used in everyday writing — only the Aleutian portion of Alaska that shares the zone actually switches.
• Arizona uses MST year-round. During the DST months, most of Arizona is effectively on the same wall clock as Pacific Daylight Time (PDT) — but the correct abbreviation is still MST, because Arizona is in the Mountain zone and just doesn't shift.
• The Navajo Nation does shift. Within Arizona, the Navajo Nation observes MDT during DST and MST during standard time. The Hopi Reservation, which is enclosed by the Navajo Nation, follows the rest of Arizona and stays on MST.

EST versus EDT: a worked example

Imagine a New York office sends a calendar invite for "9:00 a.m. EST" on June 15. Strictly speaking, this is wrong, because on June 15 New York is on EDT, not EST. Calendar software typically interprets this either as 9:00 a.m. local time in New York (which is what the sender meant) or as 9:00 a.m. UTC−5 (which would actually be 10:00 a.m. local time in New York during DST). Depending on the software, the meeting can shift by an hour for some attendees but not others.

The clean fix is one of:

• Write 9:00 a.m. ET — unambiguous because ET means "whatever clock New York is currently on."
• Write 9:00 a.m. EDT — correct for June, and unambiguous because it pins down both the zone and the DST status.
• Write 9:00 a.m. America/New_York in software — uses the IANA zone identifier, which is what calendar systems prefer internally.

Decision criteria: which form to use

A short rule that covers nearly every situation:

• Casual writing or unclear DST status → use the two-letter form (ET, CT, MT, PT, AKT, HST).
• Specifying a winter date that needs precision → use the standard form (EST, CST, MST, PST, AKST).
• Specifying a summer date that needs precision → use the daylight form (EDT, CDT, MDT, PDT, AKDT). HDT applies only to the Aleutian Islands.
• Software or APIs → use IANA identifiers (America/New_York, America/Chicago, America/Denver, America/Los_Angeles, America/Anchorage, Pacific/Honolulu) — these encode DST automatically.
• Hawaii → always HST, year-round.
• Most of Arizona → always MST, year-round.

UTC, GMT, and "Zulu"

Three terms come up alongside US abbreviations:

• UTC (Coordinated Universal Time) is the global reference. US time zones are defined as offsets from UTC.
• GMT (Greenwich Mean Time) is the historical name for the same reference and, for most everyday purposes, can be used interchangeably with UTC.
• Zulu time is a military and aviation term meaning UTC; "1300Z" means 13:00 UTC. You will see Zulu in flight plans, NOTAMs, and weather forecasts, never in office calendars.

Common mistakes

• Using EST year-round. Half the year, it should be EDT. The safest fallback when you don't know the season is ET.
• Confusing CST with China Standard Time. Outside the US, CST often means China Standard Time (UTC+8). In international correspondence, write US Central Time or CT to avoid ambiguity.
• Forgetting that Arizona is MST in summer. Phoenix in July is on MST, the same wall-clock as PDT in Los Angeles. The abbreviation does not change just because the local time happens to match a different zone.
• Writing "PST" for a date in July in California. California is on PDT in July, not PST. Use PT if unsure.
• Assuming "9 AM EST = 6 AM PST" works year-round. The three-hour gap between Eastern and Pacific is constant, but the labels switch. Use ET and PT to keep the comparison stable, or use EDT/PDT in summer and EST/PST in winter.

How software handles this

Modern calendar systems, email clients, and operating systems use the IANA Time Zone Database, where each zone is identified by a region/city string like America/Los_Angeles. The IANA identifier captures the full set of rules, including historical DST changes, so any timestamp tagged with one of these identifiers is unambiguous regardless of when it falls in the year. If you write timezone-aware code, IANA identifiers are the right tool; three-letter abbreviations are best reserved for prose.

For a quick interactive check, the converter on the homepage handles the math for any two US zones and uses the IANA identifiers under the hood.

Related reading

• Daylight saving time — when the switch from standard to daylight happens, and which states skip it.
• Time zones by state — which abbreviation applies in each US state.
• US territories — abbreviations used in Puerto Rico, the Virgin Islands, Guam, and the rest.
• A short history — why the US ended up with the current naming system.