$timezone = new DateTimeZone('America/New_York'); $transitions = $timezone->getTransitions( strtotime('January 1st 2021'), strtotime('December 31st 2021') );The returned $transitions array contains various information about each transition, including the timestamp when the transition occurred, the offset, and the abbreviation used for the timezone at that particular time. This method is part of the PHP DateTime extension, which is included in the core PHP package.