Length of the tropical year, defined as the average interval between vernal equinoxes. This calendar year was the objective of the Gregorian calendar reform, which finalized the calendar as we use it today.				365 days, 5 hours, 49 minutes (365.2424 Universal days)
Lengthening of the vernal equinox year over the last two millennia				About 10 seconds (0.0001 universal days per year)
Variation of this length in the next few millennia				less than 5 seconds
Lunar month in 2000 C.E.				29 days, 12 hours, 44 minutes, 2.9 seconds
The earliest known date				4236 B.C.E., the founding of the Egyptian calendar
Ancient Egyptian calendar year				365
Date Emperor Huangdi invented the Chinese calendar (legend)				2637 B.C.E.
Early Chinese year				354 days (lunar year) with days added at intervals to keep the Chinese lunar calendar aligned with the seasons
Early Greek year				354 days, with days added
Jewish Year				354 days, with days added
Early Roman year				304 days, amended in 700 C.E. to 355 days
The year according to Julius Caesar (The Julian calendar)				365 1/4 days
Date Caesar changed Roman year to Julian calendar				January 1, 45 B.C.E
Time the old Roman calendar was misaligned with the solar year as designated by Caesar				80 days
Total length of 45 B.C.E., known as the "Year of Confusion," after adding 80 days				445 days
Date Sanhedrin president Hillel II codified the Jewish calendar				ca. C.E. 359
The year as amended by Pope Gregory XIII (Gregorian calendar year)				365 days, 5 hours, 49 minutes, 12 seconds
Date Pope Gregory reformed the calendar				1582
Length of time the Julian calendar overestimates our calendar year per year, as determined by Pope Gregory				10 minutes 48 seconds
Days Pope Gregory removed to correct the calendar's drift				10 days
Dates Gregory eliminated by Papal bull to realign his calendar with the solar year				October 5-14, 1582
Dates most Catholic countries accepted the Gregorian calendar				1582-1584
Date Protestant Germany accepted the Gregorian calendar				partial acceptance in 1700, full acceptance in 1775
Date Great Britain (and the American colonies) accepted the Gregorian calendar				1752
Date Benjamin Franklin first proposed Daylight Saving Time				1784
Days eliminated by the British Parliament to realign the old calendar (Julian) with the Gregorian calendar				11 days
Dates Parliament eliminated				September 3-13, 1752
Date Japan accepted the Gregorian calendar				1873
Date Russia accepted the Gregorian calendar				1917 (and again in 1940)
Date China accepted the Gregorian calendar				1949
Date the Eastern Orthodox Church last voted to reject the Gregorian calendar and retain the Julian calendar				1971
Length of time the Gregorian calendar is off from the average vernal equinox year				about 12 seconds per year
Length of time the Gregorian calendar has become misaligned with the vernal equinox over the 414 years since Gregory's reform in 1582			1 hour and 20 minutes
When the Gregorian calendar will become twelve calendar hours ahead of the astronomer's mean tropical year			4th or 5th millennium C.E.
When the Gregorian calendar will become twelve calendar hours ahead of the mean vernal-equinox year			beyond the 7th millennium C.E.
Date Atomic Time replaced Earth Time as the world's official scientific time standard			1972
Current official definition of the second			time it takes for 9 192 631 770 oscillations of the Cesium atom at zero magnetic field
The mean vernal equinox year expressed in oscillations of atomic cesium at the year 2000			290 091 329 207 984 000

Notes:

Slowing of the vernal equinox year

The length of the year has increased slightly over the millennia for a variety of reasons. These include: the gradual slowing of the Earth's rotation, slow changes in the Earth's orbit due to other planets and the moon, as well as regular effects due to precession of the Earth's axis of rotation every 26,000 years.

Measures of the year

There is a subtle but important difference in two primary measures of the year, used by our calendar and by astronomers. The year mentioned above is the length of the tropical year defined as the mean interval between vernal equinoxes (1582-2000 C.E.) : 365 days, 5 hours, 49 minutes (365.2424 Universal days). Another measure of the year often used is the astronomer's mean tropical year, defined as 365 days, 5 hours, 48 minutes, 45 seconds.

Atomic time

The measurement of time is currently determined by an international consortium based in France which averages the time from approximately 220 atomic clocks in over two dozen countries. The atomic clock is the only object that both tells time and generates a precise time scale.

Historically, the calculation of time has been based on the position of the earth relative to the sun using noon, when the sun is highest in the sky, as a marker. The length of the second, which corresponds to the length of time required for 9,192,631,770 cycles of the Cesium atom at zero magnetic field, was determined near the end of the 19th century; this second is thus equivalent to the second defined by the fraction 1/31 556 925.97 47 of the year 1900. In 1967, the official second was set as equal to an average second of Earth's rotation time; the calculation of the average is necessary due to the fact that the earth rotates at a slightly irregular rate.

Today, time is determined by counting official seconds. This is subject to slight measurement inaccuracies; thus, the international community calculates a stable time by averaging accumulated seconds from several clocks worldwide. Next, this figure is compared to a few highly accurate laboratory measurements of the second. Every month, the official world time is adjusted by a few nanoseconds. Politically, time is a cooperative venture; and, by making time an international endeavor, the international community benefits from the combined resources of many laboratories.

Leap seconds in universal time coordinated (UTC)

World time is typically adjusted every year by adding what is called a "leap second." Because the time calculated by the position of the sun differs from the time calculated by the atomic standard, it is occasionally necessary to adjust international time standards to match the position of the Earth.

The rotational speed of the Earth changes slightly for several reasons, some of which are not fully understood. Large scale movements of water and changes in the atmosphere affect the Earth's angular momentum. Tidal friction from the moon, which results in the rise of tides in the ocean, diminishes the speed of rotation. Physical processes occurring on or within the Earth also affect the earth's rotation.

Information supplied by: http://webexhibits.org/calendars