It was a refinement to the Julian calendar involving an approximately 0.002% correction in the length of the calendar year. The motivation for the reform was to stop the drift of the calendar with respect to the equinoxes and solstices—particularly the northern vernal equinox, which helps set the date for Easter. Transition to the Gregorian calendar would restore the holiday to the time of the year in which it was celebrated when introduced by the early Church. The reform was adopted initially by the Catholic countries of Europe. Protestants and Eastern Orthodox countries continued to use the traditional Julian calendar and adopted the Gregorian reform, one by one, after a time, at least for civil purposes and for the sake of convenience in international trade. The last European country to adopt the reform was Greece, in 1923. Many (but not all) countries that have traditionally used the Julian calendar, or the Islamic or other religious calendars, have come to adopt the Gregorian calendar for civil purposes.
The Gregorian reform contained two parts: a reform of the Julian calendar as used prior to Pope Gregory XIII's time, and a reform of the lunar cycle used by the Church with the Julian calendar to calculate the date of Easter. The reform was a modification of a proposal made by Aloysius Lilius, who proposed to reduce the number of leap years that occur in every four centuries from 100 to 97, by making 3 out of 4 centurial years common years instead of leap years. Lilius also produced an original and practical scheme for adjusting the epacts of the moon when calculating the annual date of Easter, solving a long-standing obstacle to calendar reform.
The Gregorian reform modified the Julian calendar's scheme of leap years as follows:
Every year that is exactly divisible by four is a leap year, except for years that are exactly divisible by 100, but these centurial years are leap years if they are exactly divisible by 400. For example, the years 1700, 1800, and 1900 are not leap years, but the year 2000 is.
In addition to the change in the mean length of the calendar year from 365.25 days (365 days 6 hours) to 365.2425 days (365 days 5 hours 49 minutes 12 seconds), a reduction of 10 minutes 48 seconds per year, the Gregorian calendar reform also dealt with the accumulated difference between these lengths. The canonical Easter tables were devised at the end of the third century, when the vernal equinox fell either on 20 March or 21 March depending on the year's position in the leap year cycle. As the rule was that the full moon preceding Easter was not to precede the equinox, the date was fixed at 21 March for computational purposes and the earliest date for Easter was fixed at 22 March. The Gregorian calendar reproduced these conditions by removing ten days.
To unambiguously specify a date, dual dating or Old Style and New Style dates are sometimes used. Dual dating gives two consecutive years for a given date because of differences in the starting date of the year or to give both the Julian and the Gregorian dates. The "Old Style" (O.S.) and "New Style" (N.S.) notations indicate either that the start of the Julian year has (or has not) been adjusted to start on 1 January (even though documents written at the time use a different start of year), or that a date conforms to the (old) Julian calendar rather than the (new) Gregorian.
The Gregorian calendar continued to use the previous calendar era (year-numbering system), which counts years from the traditional date of the nativity (Anno Domini), originally calculated in the 6th century by Dionysius Exiguus. This year-numbering system, also known as Dionysian era or Common Era, is the predominant international standard today.