Daylight saving time

Daylight saving time (DST), also referred to as daylight savings time, daylight time (United States and Canada), or summer time (United Kingdom, European Union, and others), is the practice of advancing clocks to (in theory) make better use of the longer daylight available during summer by having darkness fall at a later clock time. The typical implementation of DST is to adjust clocks ahead of standard time by one hour in spring or late winter, and to set clocks back by one hour in the autumn (or fall in North American English, hence the mnemonic: "spring forward and fall back").

In many countries, the part of the year when DST is observed is much longer than the part aligned to standard time. The practice of DST was first tested and widely adopted in the early 20th century, since which many countries have abandoned DST partially or entirely.

Overview

As of 2023, around 34 percent of the world's countries use DST, primarily in Europe and North America, with some countries observing it only in some regions. Six of the ten provinces and one of the three territories in Canada observe DST, with some regional exceptions. It is observed by four Australian states and one territory. In the United States, it is observed everywhere except the states of Hawaii and Arizona (within the latter, however, the Navajo Nation does observe it) and the territories of the US.

Historically, there is evidence that ancient societies adopted seasonal changes to their timekeeping to make better use of daylight. However, these were changes to the time divisions of the day rather than setting the whole clock forward. In a satirical letter to the editor of the Journal de Paris in 1784, Benjamin Franklin suggested that if Parisians could only wake up earlier in the summer they would economize on candle and oil usage, but he did not propose changing the clocks.