Atlantic Standard Time: geographic map, Caribbean boundaries, and seasonal shifts
Sitting one hour ahead of Eastern Time, Atlantic Standard Time spans Canada's Maritime provinces, Puerto Rico, and the Caribbean. Here is where UTC−4 applies, which regions observe Daylight Saving Time, and why half the zone never touches their clocks.
The short answer
Atlantic Standard Time (AST) is UTC−4 — one hour ahead of Eastern Standard Time (EST, UTC−5). Canada’s Maritime provinces and Bermuda switch to Atlantic Daylight Time (ADT, UTC−3) from March to November, while Puerto Rico, the US Virgin Islands and the Eastern Caribbean stay on AST (UTC−4) year-round.
When people in North America think of primary continental time zones, they usually name four: Eastern, Central, Mountain, and Pacific.
Yet further east sits a vital fifth zone: Atlantic Standard Time (AST).
Operating at UTC−4, Atlantic Time encompasses Canada's vibrant Maritime provinces and extends thousands of miles south across Bermuda, Puerto Rico, and the Caribbean archipelago.
What makes Atlantic Time tricky for international teams and travelers is that the northern and southern halves of the zone handle Daylight Saving Time in completely opposite ways. Here is the full geographic breakdown, how the offsets shift throughout the year, and who stays fixed at UTC−4 permanently.
Direct Answer: What Is Atlantic Standard Time?
Atlantic Standard Time (AST) is the civil time zone located at UTC−4 (four hours behind the zero meridian). It sits exactly one hour ahead of Eastern Standard Time (EST / UTC−5).
- In Canada (Nova Scotia, New Brunswick, Prince Edward Island), AST is observed during the winter months, shifting to Atlantic Daylight Time (ADT / UTC−3) in the summer.
- In the Caribbean (Puerto Rico, the US Virgin Islands, the Dominican Republic), AST is observed year-round without Daylight Saving Time.
Geographic Coverage: Who Lives on Atlantic Time?
| Territory / Country | Standard Time (Winter) | Summer Observance | Daylight Saving Time? |
|---|---|---|---|
| Nova Scotia (Canada) | UTC−4 (AST) | UTC−3 (ADT) | Yes (March to November) |
| New Brunswick (Canada) | UTC−4 (AST) | UTC−3 (ADT) | Yes (March to November) |
| Prince Edward Island (Canada) | UTC−4 (AST) | UTC−3 (ADT) | Yes (March to November) |
| Labrador (Canada, most areas) | UTC−4 (AST) | UTC−3 (ADT) | Yes (March to November) |
| Bermuda (UK Overseas Territory) | UTC−4 (AST) | UTC−3 (ADT) | Yes (March to November) |
| Puerto Rico (US Territory) | UTC−4 (AST) | UTC−4 (AST) | No (Permanent UTC−4) |
| US Virgin Islands (USVI) | UTC−4 (AST) | UTC−4 (AST) | No (Permanent UTC−4) |
| Dominican Republic | UTC−4 (AST) | UTC−4 (AST) | No (Permanent UTC−4) |
| Barbados & Eastern Caribbean | UTC−4 (AST) | UTC−4 (AST) | No (Permanent UTC−4) |
Why Do Puerto Rico and New York Share the Same Clock in Summer?
The most common point of confusion with Atlantic Time occurs between New York and San Juan, Puerto Rico:
In Winter (November to March):
- New York observes Eastern Standard Time (EST / UTC−5).
- Puerto Rico observes Atlantic Standard Time (AST / UTC−4).
- Result: Puerto Rico is 1 hour ahead of New York. When it is 12:00 PM in New York, it is 1:00 PM in San Juan.
In Summer (March to November):
- New York advances its clocks one hour to Eastern Daylight Time (EDT / UTC−4).
- Puerto Rico does not change its clocks and remains on Atlantic Standard Time (AST / UTC−4).
- Result: Puerto Rico and New York show the exact same time. Both sit at UTC−4 for eight months of the year.
This creates the illusion for tourists and business travelers that Puerto Rico "moves into Eastern Time" during the summer, when in reality Puerto Rico's clocks never moved at all.
How Does Atlantic Time Border Newfoundland's Half-Hour Zone?
Canada's Atlantic provinces adopted UTC−4 because the 60° West meridian passes directly through Nova Scotia and eastern Canada, making UTC−4 the natural solar center of the region.
However, travel just a few miles east across the Cabot Strait to the island of Newfoundland, and the clocks jump by a non-standard 30 minutes. Newfoundland operates on its own dedicated zone: Newfoundland Standard Time (NST / UTC−3:30).
When it is 10:00 AM in Halifax, Nova Scotia (AST), it is 10:30 AM in St. John's, Newfoundland.
Why Doesn't the Caribbean Observe Daylight Saving Time?
Neither Puerto Rico, the US Virgin Islands, nor the sovereign nations of the Lesser Antilles observe Daylight Saving Time.
Because these islands sit close to the equator (between 10°N and 18°N latitude), the length of daylight varies by less than two hours between the summer solstice and the winter solstice. In San Juan, the sun rises around 6:00 AM and sets around 6:30 PM year-round.
Changing clocks by an hour would offer zero energy-saving benefits and would disrupt agricultural rhythms and maritime schedules. Under the US Uniform Time Act of 1966, state and territorial legislatures are permitted to exempt themselves from Daylight Saving Time, which Puerto Rico and the USVI have done consistently.
Frequently Asked Questions
Is Atlantic Time the same as Eastern Time?
No. Standard Atlantic Time (AST) is UTC−4, which is one hour ahead of standard Eastern Time (EST / UTC−5). However, during North American Daylight Saving Time (March to November), Eastern Daylight Time (EDT) shifts to UTC−4, temporarily matching AST.
Does Bermuda observe Daylight Saving Time?
Yes. Although located far south of the Canadian Maritimes at 32°N latitude, Bermuda follows North American Daylight Saving Time rules, springing forward to ADT (UTC−3) in March and falling back to AST (UTC−4) in November.
What is the time difference between Atlantic Time and London?
During northern winter, London is at UTC+0 and Atlantic Standard Time is at UTC−4, creating a 4-hour difference (12:00 PM in Halifax is 4:00 PM in London). In summer, when Canada is on ADT (UTC−3) and the UK is on British Summer Time (UTC+1), the difference remains 4 hours.
Does any part of the US mainland use Atlantic Time?
No. No state on the continental United States observes Atlantic Time. Maine and parts of New England have occasionally debated moving from Eastern Time to Atlantic Time to avoid early winter sunsets, but all proposals have stalled in state legislatures.
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Figures on this page are computed from the IANA time zone database. Last generated 2026-11-02.