Atlantic Daylight Time is the Atlantic Time Zone of the United States of America (USA) and Canada. Atlantic Standard Time (AST) is 4 hours behind Greenwich Mean Time (GMT-4).
In most states in the USA and in most provinces in Canada, Daylight Saving Time (DST) is observed. During DST the time is shifted forward by 1 hour to ADT; which is 3 hours behind Greenwich Mean Time (GMT-3).
After the Summer months Atlantic Time is shifted back by 1 hour to Atlantic Standard Time (EST) or (GMT-4).