Atlantic Time Zone

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.