How daylight saving time works

Daylight saving time (DST) moves local clocks, usually by one hour, for part of the year. Not every country uses it, and those that do may change on different dates. That is why London and New York are often five hours apart but can be four hours apart for a few weeks in March.

PHP reads DST from the IANA timezone database for the date you ask about. Do not subtract a remembered offset if you need an accurate comparison on a specific day.