Source: DaysPedia.Com

Daylight Saving Time

In the US, Daylight Saving Time (DST) begins Sunday, March 9, 2025 at 2:00am - the second Sunday in March. DST will end Sunday, November 2, 2025 at 2:00am - the first Sunday in November.

Around 34 percent of the world's countries use DST. Almost all states in the US use DST. DST is the practice of advancing clocks to make better use of the longer daylight available during summer so that darkness falls at a later clock time. The typical implementation of DST is to set clocks forward by one hour in spring or late winter, and to set clocks back by one hour to standard time in the autumn (hence the mnemonic: "spring forward and fall back").

In the United States, DST is observed by all states except Hawaii and Arizona. DST was first implemented in the US with the Standard Time Act of 1918, a wartime measure for seven months during World War I in the interest of adding more daylight hours to conserve energy resources. Year-round DST, or "War Time", was implemented again during World War II. After the war, local jurisdictions were free to choose if and when to observe DST until the Uniform Time Act which standardized DST in 1966.

The law does not require that all states observe DST. Individual states may exempt themselves from DST and observe standard time year-round by passing a state law, providing certain requirements are met. Arizona has not observed daylight saving since the year following the enactment of the Uniform Time Act of 1966. Native American nations within Arizona may choose. The Navajo Nation has chosen to use DST throughout its territory, which includes parts of New Mexico and Utah where DST is observed. The Hopi Nation, with territory surrounded entirely by the Arizona portion of the Navajo Nation, has chosen not to observe DST.

GMT - UTC - ZULU

Greenwich Mean Time (GMT) is the clock time at the Prime Meridian (0 degrees longitude) in Greenwich, England. Here, the word "mean" means "average." It refers to the fact that noon GMT is the moment on average each year when the sun is at its highest point in the sky at the Greenwich meridian. (Because of Earth's uneven speed in its elliptical orbit and it's axial tilt, noon GMT isn't always when the sun crosses the Greenwich meridian.)

For astronomical purposes, the GMT day was said to start at noon and run until noon the following day. For everyone else, the GMT day started at midnight. When everyone switched to the midnight-based convention in the 1920s and 1930s, this midnight-based time standard was given the new name of Universal Time. Since this change, the term GMT isn't used much anymore, except by those living in the UK and its Commonwealth countries where it's used to describe the local time during the winter months.

Coordinated Universal Time (UTC is a modern version of GMT. One of the biggest differences between GMT and UTC is that UTC does not observe Daylight Savings Time. Another name for UTC Time is "Zulu" (sometimes called military time, or "Z Time.")

The abbreviation for Coordinated Universal Time is UTC which represents a compromise between the English (Coordinated Universal Time) and the French phrase - Temps Universel Coordonne.

London observes daylight savings time called BST (British Summer Time) which is 1 hour ahead of UTC (Coordinated Universal Time). During the winter, GMT - Greenwich Mean Time(UTC+0) is in use. In the UK, the clocks go forward 1 hour at 1am on the last Sunday in March, and back 1 hour at 2am on the last Sunday in October. The period when the clocks are 1 hour ahead is called British Summer Time. There’s more daylight in the evenings and less in the mornings (sometimes called Daylight Saving Time). When the clocks go back, the UK is on Greenwich Mean Time.

For comparison, below is the current time in Coordinated Universal Time (UTC) and the current time in London.

Coordinated Universal Time (UTC)

Current Time in London UK

Midnight and Noon with 12-Hour Clock

AM stands for "Ante Meridiem," a Latin phrase meaning "before midday," while PM stands for "Post Meridiem," which translates to "after midday." This means that the hours before noon are labeled as AM, while those after noon are labeled as PM. Midnight marks the transition from one day to another, occurring at 00:00 hours in a 24-hour clock format. In the context of a 12-hour clock system, midnight is designated as 12 AM (Ante Meridiem). Noon is referred to as 12 PM.

Both midnight and noon signify important transitions: midnight marks the end of one day and the start of another, while noon represents the halfway point of the day itself. Military time (24-hour format) provides clarity; for example, instead of saying 'midnight,' you would simply say '0000' hours.

The United States' Time Zone History

The US was divided into 4 standard time zones on November 18, 1883, and jurisdiction for the zones was given to the Interstate Commerce Commission (ICC). Since 1967, the US Department of Transportation (DOT) has been responsible for governing time zones in the country.

Time zones in the USA are defined in the U.S. Code, Title 15, Chapter 6, Subchapter IX - Standard Time. The time zones in the law are defined by their offset from Coordinated Universal Time (UTC).

There are 9 official time zones according to the law. In addition the uninhabited atolls of Baker Island (AoE) and Wake Island (WAKT) add to the time zone count, making 11 the total number of time zones in the US.