1) A leap year is any year evenly divisible by four - except for century years, which have to be divisible by 400. It’s not a perfect system: The Gregorian year is 27 seconds longer than the astronomical year. By 12008, we’ll be three days off.
2) October 5-14, 1582, never happened in Catholic lands. Brits (and their American subjects) born September 3 to 13 had no birthday in 1752. Those days were dropped when the Western world switched from the Julian to the Gregorian calendar.
3) International Atomic Time - kept by ultra-precise clocks - is gradually out-pacing astronomical time, which is determined by our planet’s rotation. (Earth’s spin is slowing - what a drag.) So in 1972, the International Earth Rotation and Reference Systems Service began adding occasional leap seconds. They’ve done it 23 times, most recently adding an extra “one-Mississippi” on December 31, 2005.
(from: Wired Magazine online - www.wired.com/science/discoveries/magazine/16-02/st_3smart)
