AST UTC−4:00 DST ended 6 Nov 2016 | ADT UTC−3:00 DST begins 12 Mar 2017 | |
 | ||
AST 6:32 pm on 28 Feb 2017 | ||
The Atlantic Time Zone is a geographical region that keeps standard time—called Atlantic Standard Time (AST)—by subtracting four hours from Coordinated Universal Time (UTC), resulting in UTC-4; during part of the year some parts of it observe daylight saving time by instead subtracting only three hours (UTC-3). The clock time in this zone is based on the mean solar time of the 60th meridian west of the Greenwich Observatory.
In Canada, the provinces of New Brunswick, Nova Scotia, and Prince Edward Island reckon time specifically as an offset of 4 hours from Greenwich Mean time (GMT-4). Small portions of Quebec (eastern Côte-Nord and the Magdalen Islands) are also part of the Atlantic Standard Time Zone. Officially, the entirety of Newfoundland and Labrador observes Newfoundland Standard Time, but in practice most of Labrador uses the Atlantic Standard Time Zone.
No portion of the continental United States is located in the Atlantic Time Zone; however the territories of Puerto Rico and the U.S. Virgin Islands do fall under Atlantic Standard Time.
Those portions of the Atlantic Time Zone that participate in daylight saving time do so as Atlantic Daylight Time (ADT), which has one hour added to make it only three hours behind GMT (UTC-3).