The 1918 shift that moved an hour of daylight from morning to evening

Wars & Conflicts · Published 16 October 2026 · Updated 25 October 2026 Swipe more like this →

In short: Most Americans assume daylight saving time is an ancient tradition, but it was formally introduced in the United States in 1918. While it was originally designed to maximize evening sun, modern rules now extend the practice to cover about 65% of the year.

Daylight saving time (DST) functions by advancing clocks one hour in the spring and moving them back in the fall. By moving one hour of sunlight from dawn to dusk, this change provides extra daylight during the hottest periods. Current American regulations require that timepieces advance by one hour at 2:00 a.m. this occurs on March's second Sunday, while clocks revert at 2:00 a.m. on the initial Sunday of November.

The modern duration of DST was significantly altered by the Energy Policy Act of 2005. This legislation increased the length of the period by about one month to help reduce energy consumption. As a result, DST is now in effect for 238 days—roughly 65% of the year. While the government retains the right to revert to previous laws if these changes prove unpopular or ineffective, the 2005 act remains the current framework for the transition.

The practice is not universal. While many regions adopt these changes, Arizona (excluding the Navajo Indian Reservation) along with Puerto Rico, the Virgin Islands, Guam, American Samoa, and Hawaii stay excluded. Furthermore, the Uniform Time Act of 1966 established the uniform set of rules for states that choose to opt into the system. Despite these established rules, legislative efforts to make DST permanent, such as the Sunshine Protection Act, have faced hurdles in the federal approval process.

Technologically, the burden of these shifts falls largely on operating systems. Most internet time services, like the Network Time Protocol (NTP), do not contain DST information; they rely on the user's device to determine the correct time. This means that hardware like 'smart' clocks manufactured before the 2007 rule changes may still operate on outdated schedules, potentially requiring manual adjustment to stay accurate.

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