Time Zones in Canada and U.S.A.
The Atlantic Time Zone or Atlantic Standard Time (AST) stands for time, which is four hours behind the Greenwich Mean Time (UTC/GMT -4). This zone is based on the mean solar time of the 60th meridian west of the Greenwich Observatory.
Atlantic Daylight Time (ADT) is observed when the clocks are adjusted to daylight saving time. This is during the spring, summer and autumn months. Atlantic Daylight Time is three hours behind the Greenwich Mean Time (UTC/GMT -3).
Areas, which follow the Atlantic Time Zone and which do not apply daylight saving time, there Atlantic Standard Time is used also during the summer months. This is for example in the Virgin Islands of the United States. Atlantic Standard Time is commonly shortened to Atlantic Time (AT).
In Canada, the provinces of New Brunswick and Nova Scotia are in the Atlantic Time Zone. Prince Edward Island and small portions of Quebec (eastern Côte-Nord and the Magdalen Islands) are also following the Atlantic Standard Time, as well as the Virgin Islands of the United States.
Effective 2007, the local time changes from AST to ADT at 02:00 LST to 03:00 LDT on the second Sunday in March, and returns at 02:00 LDT to 01:00 LST on the first Sunday in November.
Greenwich Mean Time (GMT) is originally referring to mean solar time at the Royal Observatory in Greenwich, London. GMT and Coordinated Universal Time (UTC) can be seen equivalent when fractions of a second are not important.