In 1582 Pope Gregory XIII (1502–1585, r. 1572–1585) consulted with astronomers and decreed another change in the calendar to remove the eleven minutes and fourteen seconds’ difference between the Julian year’s length and Earth’s orbital period around the Sun (which is very close to 365.2422 days). First, the Gregorian calendar reset the date ahead by ten days, so that the first day of spring would be March 21 of each year. Then, it reduced the number of leap-year days by three days every four centuries. This was accomplished by modifying the leap-year rule: if a year is divisible by four, it would be a leap-year, unless the year is also divisible by one hundred. If a year is divisible by one hundred, it would only be a leap-year if it is also divisible by four hundred. That means that the year 1600 and the year 2000 were both leap years, but the years 1700, 1800, and 1900 were not—and the years 2100, 2200, and 2300 will not be leap years either.

The Gregorian calendar is the basis of the modern calendar. It is accurate to within twenty-six seconds per year, on average (0.0003 days). To keep everything on track over the long term, by international agreement every once in a while a leap-second is added to the end of a year. The International Telecommunications Union, however, has recently begun to consider the possibility of ending the practice of adding leap-seconds because modern timekeeping makes such adjustments unnecessary.