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)
The earliest known calendar date - 4236 B.C., the founding of the Egyptian calendar
Ancient Egyptian calendar year - 365 days
Date Emperor Huangdi invented the Chinese calendar (legend) - 2637 B.C.
Early Chinese year - 354 days (lunar year) with days added at intervals to keep the Chinese lunar calendar aligned with the seasons
Leaping ahead ...
Upcoming Leap Days
2016 - Monday
2020 - Saturday
2024 - Thursday
2028 - Tuesday
2032 - Sunday
Early Greek year - 354 days, with days added
Jewish Year - 354 days, with days added
Early Roman year - 304 days, amended in 700 A.D. to 355 days
The year according to Julius Caesar (The Julian calendar) - 365 days
Date Caesar changed Roman year to Julian calendar - Jan. 1, 45 B.C.
Time the old Roman calendar was misaligned with the solar year as designated by Caesar - 80 days
Total length of 45 B.C., known as the "Year of Confusion," after adding 80 days - 445 days
Date Sanhedrin president Hillel II codified the Jewish calendar - ca. A.D. 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 - Oct. 5-14, 1582
Date Benjamin Franklin first proposed Daylight Saving Time - 1784
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 12 calendar hours ahead of the astronomer's mean tropical year - 4th or 5th millennium A.D.
Date Atomic Time replaced Earth Time as the world's official scientific time standard - 1972