Which calendar is used for the modern international date of New Year's Eve on December 31?

The Gregorian calendar is used for the modern international date of New Year's Eve on December 31.

Pope Gregory XIII introduced the calendar in 1582 to correct the accumulated drift between the calendar year and the solar year. The reform adjusted leap-year rules: century years are not leap years unless divisible by 400.

The Gregorian calendar was adopted at different times by different countries, so December 31 was not universally used at once. Today it is the civil calendar used internationally for most official and commercial purposes, although other calendars remain important for religious and cultural observances.

The Julian calendar is an important historical predecessor and is still used by some churches for liturgical dates. Confusing the two calendars can shift a date, but the globally standard civil New Year's Eve is December 31 in the Gregorian system.