Today's also a good day to change the batteries in your smoke detector, which you should do twice a year.

DST commonly begins in the Northern Hemisphere on either the first Sunday in April or the last Sunday in March, and ends on the last Sunday in October. In the Southern Hemisphere, the beginning and ending dates are switched (thus the time difference between, e.g., the United Kingdom and Chile may be three, four, or five hours).

Chile switches to DST at 24:00 on the second Saturday in October and reverts to LST at 24:00 on the second Sunday the following March. The current law which affects the entire country was enacted in 1970, but it had observed the practice as early as 1927 when the country had been divided into two distinct time zones. In specific years the starting and ending dates have been modified for political or climactic reasons.

North America generally follows the same procedure, going by local time in each zone, each time zone switching at 02:00 LST (local standard time) to 03:00 LDT (local daylight time) on the first Sunday in April, and again from 02:00 LDT to 01:00 LST on the last Sunday in October. The Canadian province of Newfoundland and Labrador is an exception in that the time changes take place at 00:01 local standard time and 00:01 local daylight time respectively. Also, in 1990, they experimented with Double Daylight Time, when the clocks went ahead by two hours, instead of the usual one hour. The Energy Policy Act of 2005, signed by President George W. Bush, will extend DST, which might prompt neighboring countries with integrated economies and schedules (especially Canada and Mexico) to adopt these changes as well. The Canadian province of Ontario has already pledged to change its daylight savings rules to match the new US rules.

All countries in Europe, except Iceland as already noted, observe daylight-saving time and change on the same date: moving clocks forward one hour on the last Sunday in March and back one hour on the last Sunday in October. In the West European (UTC), Central European (UTC+1), and East European (UTC+2) time zones the change is simultaneous: on both dates the clocks are changed everywhere at 01:00 UTC, i.e. from local times of 01:00/02:00/03:00 to 02:00/03:00/04:00 in March, and vice versa in October. (See also: European Summer Time). In Russia, however, although the changeover dates are the same, clocks are moved forward or back at 02:00 local time in all zones. Thus in Moscow (local time = UTC+3 in winter, UTC+4 in summer), daylight-saving time commences at 05:00 UTC on the last Sunday in March, and ends at 06:00 UTC on the last Sunday in October.