• What is history of clocks, calendars and Daylight Saving Time?
• When does Daylight Saving Time/Standard Time begin?
• What is the origin of hours, minutes and seconds?
• Who regulates time zones?

What is the history of. . .?

For information about the history of clocks, watches, calendars, daylight saving time, and variety of other topics, please visit the historical exhibits page.

When does Daylight Saving Time/Standard Time begin?

Daylight Saving Time (DST) begins each year at 2:00 a.m. (local time) on the second Sunday in March in most of the United States and its territories. Clocks must be moved ahead one hour when DST goes into effect. DST is not observed in Hawaii, American Samoa, Guam, Puerto Rico, the Virgin Islands and the state of Arizona (with the exception of the Navajo Indian Reservation, which does observe DST).

Standard Time begins each year at 2:00 a.m. (local time) on the first Sunday of November. Move your clocks back one hour at the resumption of Standard Time.

In 2008, DST is from 2:00 a.m. (local time) on March 9th until 2:00 a.m. (local time) on November 2nd.

In 2009, DST is from 2:00 a.m. (local time) on March 8th until 2:00 a.m. (local time) on November 1st.

On August 8, 2005, President Bush signed the Energy Policy Act of 2005, which included the changes in Daylight Saving Time described above, effective in 2007. Prior to 2007, DST began at 2:00 a.m. (local time) on the first Sunday in April, and ended at 2:00 a.m. (local time) on the last Sunday in October. The new rules for DST beginning in 2007 mean an extra four or five weeks of DST each year. There will now be a total of 238 days of DST, compared to a total of 210 days of DST in 2006 under the previous rules, and the U. S. will remain on DST for about 65% of the year.

NIST broadcasts from radio station WWVB are used to synchronize many millions of wall clocks, watches, clock radios, and other timekeeping devices. The NIST radio broadcasts include information on whether or not DST is in effect, so radio controlled clocks will adjust whenever we transition between DST and ST. For more information, see the NIST web page on radio controlled clocks.

What is the origin of hours, minutes and seconds?

A sundial described in 1300 BCE reveals that the Egyptians determined a daily cycle to be made up of ten hours of daylight from sunrise to sunset, two hours of twilight and twelve hours of night. Their calendar year was divided into 36 decans, each ten days long, plus five extra days, totaling to a 365 day year. Each decan was equivalent to a third of the zodiacal sign and was represented by a decanal constellation. The night corresponded to about twelve decans, half a day to eighteen decans. Similar to the system used in Oriental clocks, the night was thus divided into twelve hours, with seasonable variations of the hour's length. Later, Hellenistic astronomers introduced equinoctial hours of equal length.

The Babylonians (in about 300-100 BCE) performed astronomical calculation in the sexagesimal (base-60) system. This was extremely convenient for simplifying time division, since 60 is divisible by 2, 3, 4, 5, 6, and 10. What we now call a minute derives from the first fractional sexagesimal place; the second fractional place is the origin of the second.

Who regulates time zones?

Time zones were originally the responsibility of the Interstate Commerce Commission. The need for time zones was first recognized by the railroads when they made it possible to transport goods and travelers across the country faster than ever before. The four original time zones (Eastern, Central, Mountain and Pacific) were established on November 18, 1883. In 1967, a congressional act transferred the duties of the Interstate Commerce Commission to the Department of Transportation (DOT). The DOT (and not NIST) is also responsible for the rules governing Daylight Saving Time.

For more details, see the United States Law on Standard Time. 15 U.S.C. §6(IX)(260-7)