He also changed the rule for leap years. In the Julian Calendar, a year is a leap year if it's divisible by four. In the Gregorian Calendar, a year is a leap year if either it's divisible by 4 but not
by 100 or it's divisible by 400. In other words, a year which is divisible by 4 is a leap year
unless it is divisible by 100 but not by 400. Thus the years 1600 and 2000 are leap years, but 1700, 1800, 1900 and 2100 aren't.
But simplicity still didn't reign. The date of the vernal equinox in the calendar of the Roman Catholic Church as established by the Council of Nicea in 325 A.D. is March 21, but the effect of removing ten days in 1582 had the result that the vernal equinox occurs in the Gregorian Calendar mostly on March 20, less often on March 21, sometimes on March 19 and sometimes even on March 22 according to local time in the Far East. So should Pope Gregory have omitted nine days? Or perhaps eleven? Presumably Pope Gregory's astronomical advisors considered all three possibilities. Some say that the choice of 10 was a compromise, saying that the omission of 10 days made it easier to correct old calendars simply by the insertion of an "X," the Latin numeral for "10."
In many countries, the general population used the Julian Calendar long after the official introduction of the Gregorian Calendar. Thus events were recorded in the 16th to 18th Centuries on varying dates, depending on which calendar was used. Dates recorded in the Julian Calendar were marked "O.S." for "Old Style," and those in the Gregorian Calendar were marked "N.S." for "New Style."
Sweden didn't adopt the Gregorian Calendar until 1753, Japan in 1873, Egypt in 1875, Eastern Europe during 1912 to 1919 and Turkey not until 1927. Following the Bolshevik Revolution in Russia, the new Communist government decreed that 13 days would be omitted from the calendar, the day following January 31, 1918, O.S. becoming February 14, 1918, N.S.
Every date recorded in history prior to October 15, 1582 (Gregorian), such as the coronation of Charlemagne as Holy Roman Emperor on Christmas day in the year 800, is a date in the Julian Calendar, since on those dates the Gregorian Calendar hadn't yet been invented. Dates after October 15, 1582 (Gregorian) have equivalent, but different, dates in the Julian Calendar.
Thus, any day in the history of the Earth, either in the past or in the future, can be specified as a date in either of these two calenders. The dates will generally be different; in fact they'll be the same only for dates from March 1st, 200, to February 28, 300. The dates in neither calendar will coincide with the seasons in the distant past or distant future, but that doesn't affect the validity of these calendars as systems for uniquely identifying particular days.
For genealogists, mankind's predisposition for fooling around with time can throw roadblocks into their search for relatives. It's only by understanding all the nuances of time manipulation that a genealogist can hope to find the right person in the right time.