Why one zone gets two labels

Washington sits on Eastern Time. In January its clocks read EST. In July they read EDT. Same city, same longitude, different abbreviation. The split exists because most of the US moves its clocks forward for part of the year. From the second Sunday in March to the first Sunday in November, clocks run one hour ahead. The rest of the year they run on the baseline offset.

ET, CT, MT, PT are the labels that work in any season. They mean "the local time in this zone, whatever it is right now." Use them when the DST status does not matter to what you are writing.

EST, CST, MST, PST are the baseline labels. They apply only when clocks are not advanced. In Washington, that period starts on the first Sunday of November and ends on the second Sunday of March.

EDT, CDT, MDT, PDT are the advanced labels. They apply only when DST is active. In Washington, that period starts on the second Sunday of March and ends on the first Sunday of November.

Write "EST" in July and you are wrong. Write "ET" in July and you are correct, because ET covers both states of the clock.

Eastern: ET, EST, EDT

Eastern Time covers the US east coast, including New York, Washington, Boston, and most of Florida and Georgia.

  • EST (UTC−5): the baseline, early November to early March.
  • EDT (UTC−4): the advanced label, early March to early November.
  • ET: safe in any month.

On 15 January 2026, Washington was on EST. On 15 July 2026, it was on EDT. The gap to London stays at 5 hours in both January and July. The gap to Dubai shifts: 9 hours ahead of Washington in January, 8 hours ahead in July, because Dubai does not move its clocks.

Central: CT, CST, CDT

Central Time covers Chicago, Dallas, Houston, and most of the Midwest and Plains states.

  • CST (UTC−6): the baseline.
  • CDT (UTC−5): the advanced label.
  • CT: safe in any month.

When Washington is on EST (UTC−5), Chicago is one hour behind on CST (UTC−6). When Washington moves to EDT (UTC−4), Chicago moves to CDT (UTC−5). The gap stays at one hour.

Mountain: MT, MST, MDT

Mountain Time covers Denver, Salt Lake City, and parts of the Mountain West. Phoenix sits in this zone but plays by its own rule.

  • MST (UTC−7): the baseline.
  • MDT (UTC−6): the advanced label.
  • MT: safe in any month, except when you need precision about Arizona.

Arizona exception: most of Arizona keeps MST (UTC−7) all year. From March to November, Arizona matches Pacific Daylight Time (UTC−7). From November to March, it matches Mountain Standard Time (UTC−7). Saying "MT" for Phoenix usually works, but the Navajo Nation inside Arizona does advance its clocks. The full breakdown is at /explained/arizona-mountain-time-no-dst.

Pacific: PT, PST, PDT

Pacific Time covers California, Washington, Oregon, and Nevada.

  • PST (UTC−8): the baseline.
  • PDT (UTC−7): the advanced label.
  • PT: safe in any month.

On 15 January 2026, Los Angeles sat 3 hours behind Washington (PST vs EST). On 15 July 2026, Los Angeles was still 3 hours behind Washington (PDT vs EDT). The offset holds because both zones shift on the same dates.

Alaska: AKST, AKDT

Alaska Time covers most of the state. The western Aleutian Islands are the exception.

  • AKST (UTC−9): the baseline.
  • AKDT (UTC−8): the advanced label.

Alaska observes DST. In January, Anchorage is 4 hours behind Washington. In July, it is 4 hours behind Washington. The offset holds because both zones shift together.

The far western Aleutian Islands use Hawaii-Aleutian Time (HST). Details are at /explained/alaska-time-zone-two-zones.

Hawaii: HST (no advanced variant)

Hawaii does not move its clocks. It stays on Hawaii Standard Time (UTC−10) all year.

  • HST (UTC−10): the only abbreviation for Hawaii.

Because Hawaii never shifts, its gap to the mainland changes twice a year. In January, Honolulu is 5 hours behind New York. In July, it is 6 hours behind New York.

There is no "HDT". Do not use it.

Territory abbreviations: AST, ChST, SST

Three US territories use zones that never advance their clocks.

Abbreviation Full name UTC offset Where it applies
AST Atlantic Standard Time UTC−4 Puerto Rico, US Virgin Islands
ChST Chamorro Standard Time UTC+10 Guam, Northern Mariana Islands
SST Samoa Standard Time UTC−11 American Samoa

All three stay on the same offset all year. No advanced variant exists.

Quick reference table

Year-round label Baseline label Advanced label UTC offset (baseline)
ET EST EDT UTC−5
CT CST CDT UTC−6
MT MST MDT UTC−7
PT PST PDT UTC−8
AKT (Alaska Time) AKST AKDT UTC−9
HT (Hawaii Time) HST (none) UTC−10
(none, called AST) AST (none) UTC−4
(none, called ChST) ChST (none) UTC+10
(none, called SST) SST (none) UTC−11