Which pope introduced the Gregorian calendar, whose civil New Year is January 1?

Pope Gregory XIII introduced the Gregorian calendar, whose civil New Year is January 1.

The calendar was promulgated in 1582 through the papal bull Inter gravissimas. Its main purpose was to correct the gradual drift of the Julian calendar relative to the seasons. The Julian calendar treated the year as slightly too long, causing the date of the spring equinox to move over the centuries.

The reform adjusted the leap-year rule: century years are not leap years unless they are divisible by 400. Thus 1600 and 2000 were leap years, while 1700, 1800, and 1900 were not. The reform also removed several calendar dates at its initial adoption, with the exact number depending on the country.

Catholic countries adopted the system first, while many Protestant and Orthodox countries adopted it later. Today it is the international civil standard, although some churches use other calendars for religious observances.