Which calendar replaced the Julian calendar for most civil New Year's dates in many countries?

The Gregorian calendar replaced the Julian calendar for civil New Year's dates in many countries.

Pope Gregory XIII introduced the Gregorian calendar in 1582 to correct the Julian calendar's drift relative to the seasonal year. The reform omitted several calendar dates and changed the leap-year rule so that century years are not leap years unless divisible by 400.

The Gregorian system retained January 1 as the start of the civil year in the countries that adopted it. Adoption was gradual: Catholic countries generally changed first, while Britain and its colonies adopted the calendar in 1752 and Russia adopted it for civil purposes after the 1917 Revolution.

Not every religious or cultural calendar uses January 1 as its New Year. The Julian calendar remains important in some Orthodox Christian contexts, and calendars such as the Islamic, Hebrew, and Chinese calendars determine New Year dates differently.