Julian and Gregorian Calendars

The Julian calendar, also called the Old Style calendar, is a dating system established by Julius Caesar as a reform of the Roman republican calendar. Caesar, advised by the Alexandrian astronomer Sosigenes, made the new calendar solar, not lunar, and he took the length of the solar year as 365¼ days. The year was divided into 12 months, all of which had either 30 or 31 days except February, which contained 28 days in common (365-day) years and 29 in every fourth year (a leap year, of 366 days). Because of misunderstandings, the calendar was not established in smooth operation until AD 8. Further, Sosigenes had overestimated the length of the year by 11 minutes 14 seconds, and by the mid-1500s, the cumulative effect of this error had shifted the dates of the seasons by about 10 days from Caesar’s time.
This inaccuracy led Pope Gregory XIII to reform the Julian calendar. His Gregorian calendar, also called the New Style calendar, is still in general use. Gregory’s proclamation in 1582 restored the calendar to the seasonal dates of AD 325, an adjustment of 10 days. Although the amount of regression was some
14 days by Pope Gregory’s time, Gregory based his reform on restoration of the vernal equinox, then falling on 11 March, to the date (21 March) it had in AD 325, the time of the Council of Nicaea. Advancing the calendar 10 days after 4 Oct 1582, the day following being reckoned as 15 October, effected the change.
The Gregorian calendar differs from the Julian only in that no century year is a leap year unless it is exactly divisible by 400 (e.g., 1600, 2000). A further refinement, the designation of years evenly divisible by 4,000 as common (not leap) years, will keep the Gregorian calendar accurate to within one day in 20,000 years.
See also: Poverty 1534