Which calendar is used for the official New Year's Day date of January 1 in most countries today?

The Gregorian calendar is used for the official New Year's Day date of January 1 in most countries today.

Pope Gregory XIII introduced the calendar in 1582 as a reform of the Julian calendar. Its calculation of leap years more closely matches the length of the tropical year, reducing the long-term drift between calendar dates and the seasons. The reform also adjusted the date by skipping several calendar days at its initial adoption.

Countries adopted the Gregorian system at different times. Catholic states were among the earliest adopters, while Britain and its colonies changed in 1752 and Russia adopted it for civil use after the 1917 revolution. Some religious communities and churches continue to use other calendars for particular observances.

January 1 is therefore a civil-calendar convention, not a date shared by every religious or cultural New Year. Lunar, lunisolar, and other calendars place their New Year celebrations on different dates. The Gregorian calendar is the international civil standard used for most modern business and government records.