From 2 AM EST on the 2nd Sun. of Mar. until 2 AM EDT on the 1st Sun. of Nov., the part of Florida that is UTC-5 in December switches to UTC-4, and the part that is UTC-6 in December switches to UTC-5.

It's called Daylight Saving Time in the United States, although the name Summer Time is more popular worldwide. It is usually in effect from around the vernal equinox to around the autumnal equinox in countries that are far enough away from the equator to have a considerable difference in the amount of daylight hours as the summer solstice approaches. Its purpose is to save energy by, in effect, moving an early morning hour of daylight from when many of us are asleep to the evening, when we are very busy with family and friends. The idea is to have one less hour per day in which we are using electric lighting.

Critics of the practice of changing the time on all the clocks twice per year argue that with the phasing out of incandescent lighting and the widespread use of air conditioning, we are now actually consuming more energy by having an extra hour of daylight when we're home in the evenings.

Currently part or all of 68 countries advance their clocks in or near spring (spring ahead) and turn them back to standard time in or near fall (fall back). That does not include those countries that do not change their clocks but have chosen instead to remain "permanently" on Summer Time, such as Russia.