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Spring forward moves clocks ahead, so 09:00 standard becomes 10:00 and one hour of the night is lost; autumn back does the reverse and returns it. The lost hour in spring measurably raises road accidents and cardiac events for a few days, which is why the practice remains contested.
Spring forward
adjusted = standard + shift
Autumn back
adjusted = standard - shift
No. Most of Asia, Africa and the tropics do not, since the seasonal daylight variation near the equator is small.
Convention. Lord Howe Island uses 30 minutes, and some wartime schemes used two hours of double summer time.