Which pope introduced the Gregorian calendar that determines today’s January 1 New Year’s date?

Pope Gregory XIII introduced the Gregorian calendar that determines today’s January 1 New Year’s date in most countries.

The Gregorian calendar was promulgated in 1582 through the papal bull Inter gravissimas. It corrected a gradual drift in the older Julian calendar, which had made the calendar date of the spring equinox move away from its intended position. The reform removed ten calendar days in the first countries that adopted it.

The new system also changed the leap-year rule: century years are not leap years unless divisible by 400. Thus 2000 was a leap year, while 1900 was not. This keeps the calendar closer to the length of the solar year.

Adoption was gradual rather than universal. Catholic countries changed first, while Britain and its colonies adopted the Gregorian calendar in 1752. Some churches and communities still use other calendars for religious or civil purposes.