Under its rules, every year that is evenly divided by 4 is a leap year, except for turn-of-the-century ones ending with an "00" such as 1900 or 2100, that aren't divisible by 400.

The 364-day Hanke-Henry Permanent Calendar starts every year on a Monday and, in lieu of Leap Day, offers an extra week every five or six years.

When Britain and its American colonies switched from the Julian to the Gregorian calendar in 1752, 11 days were lost. George Washington had to change his birthday from Feb. 11 to Feb. 22.