This question is kind of Euro-centric. There are other calendars in use, all around the world. We all use the same one now, because of globalization. It’s hard to build your economy when you can’t agree on what year it is. That doesn’t mean older calendars don’t exist.
The Aztecs had a very famous calendar. I believe there is a Jewish calendar. If you’ve ever heard of Chinese New Year, it’s not on January 1st because it uses a different calendar. The Japanese have calendars that start with year one, every time they get a new emperor.
Basically, the calendar based on Jesus is called the Gregorian Calendar and it was implemented by the Pope in 1582. It’s mathematically based on seasons and has all sorts of crazy tricks like leap years to keep it accurate. Most of the other calendars are lunar calendars (based on the cycles of the moon), including the one that the Gregorian one is based on.