Daylight saving time shifts the clock forward by an hour in spring and back again in autumn, so that summer evenings have more usable light. Roughly a third of the world's countries do it; most of Asia and Africa do not.
The confusing part is not the shift itself — it is that countries change on different dates. North America moves in mid-March and early November; Europe moves on the last Sundays of March and October. For a couple of weeks each year, the usual gap between New York and London is four hours instead of five.
The southern hemisphere runs opposite
Sydney and Auckland add their hour around October and remove it around April. So the distance between London and Sydney swings by two hours over the course of a year.
You do not have to track any of this
Every conversion on this site reads the current rules for each individual zone on the exact date you choose, including the transition weekends. If a government changes its rules, the underlying database is updated and the site follows automatically.