a standard time used in the zone which includes Bermuda, Nova Scotia, New Brunswick, and E Quebec, corresponding to the mean solar time of the 60th meridian west of Greenwich, England: it is four hours behind Greenwich time
See Atlantic Standard Time in American Heritage Dictionary 4
noun Abbr. AST
Standard time in the fourth time zone west of Greenwich, England, reckoned at 60° west and used, for example, in Puerto Rico and the Canadian Maritime Provinces. Also called Atlantic Time.