The Gregorian calendar is the calendar reform introduced by Pope Gregory XIII and used for January 1 New Year dates today.
Pope Gregory XIII promulgated the reform in 1582 through the papal bull Inter gravissimas. Its main purpose was to correct drift in the Julian calendar's calculation of the solar year, which had gradually displaced the date of the spring equinox used in calculating Easter.
The reform changed the leap-year rule: century years are not leap years unless divisible by 400. Thus 1600 and 2000 were leap years, but 1700, 1800, and 1900 were not. The reform also skipped dates in countries that adopted it immediately, though the number of omitted days varied by adoption date.
Catholic countries adopted the calendar first, while many Protestant and Orthodox countries adopted it later. Today it is the civil calendar used in most of the world, although other calendars remain important for religious and cultural purposes. January 1 is therefore a civil New Year date, not a universal religious New Year.