Not every country observes daylight saving time, and some regions near the equator never change clocks.

The longer version

Near the equator, daylength barely changes across the year, so there is no spare evening daylight to borrow. Shifting clocks there would only make mornings darker for no gain, which is why tropical countries generally leave their clocks alone. The Sun rises and sets at nearly the same times every day there.

Elsewhere the decision is political. Some countries adopted the practice during energy shortages and dropped it later; some regions inside a single country opt out. The result is a patchwork that changes the offset between neighbours twice a year, complicating broadcasting, transport and cross-border business. Software that tracks time zones must be updated whenever a government changes its rules.

Where this entry sits

These facts wander through clocks, calendars, time zones, and the long human effort to measure passing hours. You will find leap years, odd month lengths, ancient sundials, and the reasons modern timekeeping works the way it does. The full list sits on the Time and Calendars page, where every entry is listed in order.