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.
In the modern period, it has become customary to number the days from the beginning of the month, and 29 February is typically considered as the leap day.
For example, the years 1700, 1800, and 1900 are not leap years, but the year 2000 is.In Rome, Easter was not allowed to fall later than 21 April, that being the day of the Parilia or birthday of Rome and a pagan festival.The first day of the Easter moon could fall no earlier than 5 March and no later than 2 April.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. 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.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 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. A regular Gregorian year consists of 365 days, but as in the Julian calendar, in certain years, a leap year, a leap day is added to February.The Church of Alexandria celebrated Easter on the Sunday after the 14th day of the moon (computed using the Metonic cycle) that falls on or after the vernal equinox, which they placed on 21 March.However, the Church of Rome still regarded 25 March (Lady Day) as the equinox (until 342), and used a different cycle to compute the day of the moon.That approximation built up an error of one day every 310 years, so by the 16th century the lunar calendar was out of phase with the real Moon by four days. However, the project was interrupted by the death of Regiomontanus shortly after his arrival in Rome.European scholars had been well aware of the calendar drift since the early medieval period. The increase of astronomical knowledge and the precision of observations towards the end of the 15th century made the question more pressing.