Before standard time
Until the late 19th century, American towns set their clocks by the sun. Noon was when the sun was highest, so time changed by about one minute for every 12 or 13 miles traveled east or west. Chicago, St. Louis and Pittsburgh each had their own time, a few minutes apart.
This hardly mattered for daily life, but it was a serious problem for railroads. Each railroad company used its own reference time, so stations displayed several clocks, and timetables were confusing and sometimes dangerous.
The railroad zones of 1883
On 18 November 1883, the railroads of the United States and Canada switched to a system of standard time zones, each based on a meridian a whole number of hours from Greenwich. It became known as the day of two noons, because many clocks were reset at midday and some places saw noon twice.
The change was made by the railroads themselves, not by law. Most cities soon followed railroad time, but federal law did not yet recognize the zones.
The Standard Time Act of 1918
Congress gave standard time a legal basis with the Standard Time Act of 1918. The law:
- made standard time zones part of federal law;
- gave the Interstate Commerce Commission the power to set the boundaries between zones;
- introduced daylight saving time for the first time, as a wartime measure during World War I.
Daylight saving time was unpopular, especially among farmers, and Congress repealed the national DST rule in 1919, overriding President Woodrow Wilson’s veto. Standard time zones stayed in place, and DST became a local choice for states and cities.
War Time
During World War II, the country again changed its clocks to save energy. From February 1942 to September 1945, clocks stayed one hour ahead all year, a period known as War Time, so the zones were called Eastern War Time, Central War Time and so on.
After the war, the rules became a patchwork. Some states and cities used DST, others did not, and those that did often chose different start and end dates. Travelers and broadcasters faced constant confusion.
The Uniform Time Act of 1966
The Uniform Time Act of 1966 brought order. It set a single national DST period, from the last Sunday in April to the last Sunday in October, for any state that observed it. A state could exempt itself by law, but could not choose its own dates. Authority over time zones later passed to the new Department of Transportation.
In response to the 1973 oil crisis, Congress put the country on daylight time through the winter starting in January 1974. Dark winter mornings made the experiment unpopular, and it was cut short later that year.
Extensions of DST
The DST period has been lengthened twice:
| Years | DST starts | DST ends |
|---|---|---|
| 1967–1986 | Last Sunday in April | Last Sunday in October |
| 1987–2006 | First Sunday in April | Last Sunday in October |
| From 2007 | Second Sunday in March | First Sunday in November |
The current dates come from the Energy Policy Act of 2005, which took effect in 2007. Changes happen at 2:00 local time. See the 2026 clock change dates for the current year’s schedule.
Exceptions
Not every state observes DST. Hawaii does not, and Arizona, apart from the Navajo Nation, has stayed on Mountain Standard Time all year since 1968. Indiana long had a mix of rules by county and adopted DST statewide in 2006. US territories such as Puerto Rico, Guam and American Samoa do not use DST.
US time zones today
The continental United States uses four main zones: Eastern, Central, Mountain and Pacific. Alaska and Hawaii have their own zones, and the territories add more. Many states, including Texas, Florida and Tennessee, are split between two zones.
For current offsets and a map of the zones, see the US time zones guide, or check the local time in Eastern Time and Mountain Time.