The most popular calender in the world is the Gregorian calendar which is used by 168 countries in the world. So, January 1st is based on the Gregorian calendar but colonization has caused it to be adopted by Christian and non-christian nation.
The calendar which Pope Gregory XIII initiated in 1582 includes 365 days plus an extra day(Leap year) every four years.
Here are countries that are not yet in 2023;
1. Afghanistan and Iran
These two countries are yet to embrace the Gregorian calendar, they use the solar hijiri calendar.
The first day of Afghanistan and Iranian calendar falls on the March equinox, the first day of spring around March 21.
2. Ethiopia
The name of Ethiopia new year is Kudus yohannes. The Ethiopia new year falls on September 11 in the Gregorian calendar except the year before the leap year when it falls on September 12.
3. China
When the new moon rises between January 21 and February 20, the Chinese new year begins.
4. Nepal
In Nepal, their calendar is called Bikram Sambar. New year day by this calendar usually falls on the 14th February.