An hour shift which is seasonally inserted in some (but not all) time zones. Daylight saving time is in effect during summer, and is usually one hour ahead of standard time. In those portions of the United States which observe daylight saving time, it is in effect from the first Sunday in April to the last Sunday of October. Daylight savings time begins when clocks are set from 2:00 a.m. to 3:00 a.m. on the first Sunday in April and ends when clocks are set from 3:00 a.m. to 2:00 a.m. on last Sunday in October.

In the United States, the only states which do not use daylight saving time are Hawaii, Arizona, and most of Indiana. Since U.S. Eastern Standard Time (EST) is UT-5 hours, U.S. Eastern Daylight Time (EDT) is UT-4 hours.

While most of Indiana remains on Eastern Standard Time year-round (and is therefore has the same time as Central Daylight Time when daylight saving time is in effect, but is an hour different between October and April, at which point it has the same time as Eastern Standard Time), some portions near borders maintain the same time as the neighboring state, and therefore do shift to daylight saving time. In particular, five northwest Indiana counties (Lake, Porter, La Port, Jasper, and Newton) and five southwest Indiana counties (Gibson, Posey, Vanderburgh, Warrick, and Spencer) are part of the Central Time Zone and shift to Central Daylight Time. Meanwhile, five southeast counties (Harrison, Floyd, Clark, Dearborn, and Ohio) switch to Eastern Daylight Time on their own to stay on the same time as Cincinnati and Louisville.

In Arizona, many Indian reservations do switch to daylight saving time, while the rest of the state does not. For example, the Navajo Reservation does switch to daylight saving time, but the Hopi reservation, located in the middle of it, does not.

Alaska goes on and off daylight saving time concurrently with the rest of the U.S. However, ever since sometime in the early 1980s, Alaska has operated on Yukon Time (which is 1 hour earlier that Pacific Time). Because of this, Alaska is one hour ahead of the sun in the winter (Standard Time) and two hours ahead of the sun in the summer (DST). In Anchorage, on the summer solstice, sunset occurs just before midnight, sunrise is just after 4 a.m., and the "darkest" part of twilight is 2 a.m.

Europe also observes Daylight Saving Time, but the dates on which is goes into effect and ends are different than in the U. S. In particular, it shifts to daylight saving time one week earlier than the U.S. in the spring. In the southern hemisphere, insertion and removal are inverted from in the northern hemisphere.

Benjamin Franklin was among the first Americans to discuss the idea of Daylight Saving Time. In fact, he proposed the idea while serving as U.S. minister to France as a way of conserving the cost of lighting for shops. The purpose has been to essentially utilize more of the daylight hours in spring and summer for daytime activities. Simply put, the Sun is up longer in the sky (rises earlier and sets later) in summer. Clocks and common mechanical timepieces keep what's called civil time, which roughly corresponds to the passage of a fictitious "average" Sun through our sky. This is necessary since the Sun does not rise and set on a regular basis, but changes by as much as 15 minutes throughout the course of the year.

Daylight Saving Time was generally adopted by countries around World War I, but President Wilson repealed the United States' mandatory daylight savings law on March 31, 1918. During World War II, Daylight Savings Time was reestablished by law on a year-round basis, and later seen as a fuel saving measure during the Arab Oil Embargo crisis of 1973-74. In late 1974, standard time was brought back. During World War II, the U.S. and some other countries implemented "double" daylight savings time consisting of a two hour shift.