Editor's Note: This article is one of three sidebars to "Time to Take Note: The 1752 Calendar Change" by Kip Sperry. Read also "A Brief History of Time" by Jake Gehring and "When Is George Washington's Birthday?"

The leap-year rule of 366 days began with the Julian calendar and was later adopted by Pope Gregory XIII. At this time, one day was added to February every fourth year—leap year. The Gregorian calendar also allowed for dropping a day from every centesimal year (that is, a year ending in "00") and every year that cannot be divided by 400. If a centesimal year is divisible by 400, it is a leap year, as decreed by Pope Gregory XIII.

The century years 1700, 1800, and 1900, for example, were not leap years—each February in those years had twenty-eight days. But the year 1600 was a leap year. Today, we add one day to February every fourth year, leap year. The year 2000 was a leap year, and February had twenty-nine days because 2000 is divisible by 400.