| 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) | 365¼ 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.
