Why there are two abbreviations per zone
Each U.S. time zone has a standard form (used in winter) and a daylight form (used in summer, when daylight saving time is in effect). The names differ by one letter — S for standard, D for daylight — and the offset differs by one hour. The full rule for when each one applies is on the daylight saving time page; the short version is that daylight time runs from the second Sunday of March to the first Sunday of November, and standard time covers the rest of the year.
Hawaii and Arizona (outside the Navajo Nation) stay on standard time year-round, so they only ever use the standard abbreviation.
The full reference
| Zone | Standard | Daylight | UTC offset |
|---|---|---|---|
| Eastern | EST — Eastern Standard Time | EDT — Eastern Daylight Time | -5 / -4 |
| Central | CST — Central Standard Time | CDT — Central Daylight Time | -6 / -5 |
| Mountain | MST — Mountain Standard Time | MDT — Mountain Daylight Time | -7 / -6 |
| Pacific | PST — Pacific Standard Time | PDT — Pacific Daylight Time | -8 / -7 |
| Alaska | AKST — Alaska Standard Time | AKDT — Alaska Daylight Time | -9 / -8 |
| Hawaii–Aleutian | HST — Hawaii–Aleutian Standard Time | HDT — Hawaii–Aleutian Daylight Time | -10 / -9 |
| Samoa (American Samoa) | SST — Samoa Standard Time | — | -11 |
| Chamorro (Guam, N. Mariana Is.) | ChST — Chamorro Standard Time | — | +10 |
Hawaii itself does not use HDT; the daylight form exists in the database for the western Aleutian Islands, which are governed by the same zone but are sparsely populated.
The single-letter "neutral" form
In informal writing, you'll often see the "neutral" form — ET, CT, MT, PT, AKT, HAT — that drops the standard/daylight distinction. This is a useful shortcut: it lets you write "the show airs at 8 p.m. ET" without having to know whether it's summer or winter when the reader sees it. The cost is a small loss of precision; for most everyday purposes, it's the right choice.
Use the long form (EST, EDT, etc.) when you need to be unambiguous — for example, in a legal notice, an air-traffic context, or any document where the half-of-the-year matters.
Choosing the right abbreviation
Three rules cover almost every situation:
- If you're talking about a specific date, use the form that's correct for that date. A meeting on July 15 in New York is at, say, "10:00 a.m. EDT," not "EST." A court filing dated December 3 in Chicago is timed in "CST," not "CDT."
- If you're writing a recurring or general statement, use the neutral form. "The customer-support line is open 9 a.m.–5 p.m. PT" is correct year-round; "9 a.m.–5 p.m. PST" is only literally true for half the year.
- If you're using Arizona (outside the Navajo Nation) or Hawaii as a reference, use the standard form. Those areas don't change. "10 a.m. MST in Phoenix" is correct in any month.
Common mistakes
- "EST" used in July. Eastern Standard Time is in effect roughly November to March. In July, the correct form is EDT, or the neutral ET.
- "PST" treated as a synonym for Pacific Time. PST is specifically the standard form. If your meeting is in June, it's PDT, not PST.
- Mixing forms in one sentence. "The webinar runs 1 p.m. EST to 2 p.m. PDT" mixes a standard form with a daylight form, which doesn't describe a real moment unless the date is in the narrow window when the country is on different DST states. Pick one frame.
- Confusing CST. "CST" can mean either Central Standard Time or, in international contexts, China Standard Time (UTC+8). When writing for a global audience, write "U.S. Central Standard Time" or use the IANA identifier
America/Chicago.
A note on identifiers vs abbreviations
Software does not use the EST/EDT-style abbreviations to identify zones — it uses the longer IANA identifiers like America/New_York, America/Chicago, America/Denver, America/Los_Angeles, America/Anchorage, and Pacific/Honolulu. These identifiers cover both the standard and daylight halves of the year automatically, so a calendar entry stored as "America/New_York 14:00" will correctly resolve to EST in winter and EDT in summer. The full picture of how zones, identifiers, and offsets fit together is on the UTC and U.S. time page.
Decision rule for everyday writing
If the date is fixed, use the form that's correct for that date. If the statement is open-ended, use the neutral form (ET, CT, MT, PT). If you're writing for software, use the IANA identifier and let the library do the work.
Worked example
Imagine you're announcing a webinar for January 15. The webinar starts at 1 p.m. in your office in New York and you want a non-technical audience to know what time that means for them.
- Long-form, precise: "1 p.m. EST (10 a.m. PST, 11 a.m. MST, noon CST)."
- Neutral, equally clear: "1 p.m. ET (10 a.m. PT, 11 a.m. MT, noon CT)."
- Wrong: "1 p.m. EDT" — daylight saving time has not started yet on January 15.
- Also wrong: "1 p.m. EST" repeated for a series that runs into April — by mid-March the East Coast is on EDT.
For converting any specific time across U.S. zones in a single click, use the time converter on the homepage. For the underlying offset relationship, see UTC and U.S. time.