The Gregorian calendar is internationally the most widely used civil calendar. It is named after Pope Gregory XIII, who introduced it in October 1582. It is the calendar used in the international standard for Representation of dates and times: SO 8601:2004. It is a solar calendar based on a 365-day common year divided into 12 months of irregular lengths. 11 of the months have either 30 or 31 days, while the second month, February, has only 28 days during the common year. 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 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.