Why do the whole world have the same christian calendar?

I am talking about the whole world have the same year 2008 A.D. and everything before that is B.C. Who came out with this rule? If this like a Christian decision? So when Muslims or atheist are writing the year aren't they acknowledging Jesus?

it just a time that all know of I dont know why you find it so offensive its just the timeline that everyone can keep up with.

Who told you that the whole world uses the Christian calendar? According to the Jewish calendar, it's year 5769. According to the Chinese calendar, it's 4706. There are plenty of cultures who use a lunar calendar that don't match up with the Western BC/AD system.

true but looking into history, Europe and America have always been mostly Christain, and still are. The calendar was put into use then, and how do you expect to change something thats been used so prodominately for 2009 years? Its just a method of counting.

Yes, as 'mikeh' writes, there are a lot of other calendar systems in use.
In Nepal, where I lived and worked for many years, there at least five or six:

- Bikram Sambat, the official Nepali calendar, which begins around the Vernal Equinox each year
- then Nepal Sambat, the calendar of the Newari people, which begins at the height of the festival of Tihar (Diiwali) iin autumn
- the Western Gregorian calendar is used in dealings with foreigners
- the Gurung and Tamang peoples have their own calendar, which usually begins in late December/early January (lunar)
- and the Sherpas and Tibetans use the lunar Tibetan calendar, which starts a little later, usually in February
- finally, there is a sacred Hindu calendar used by astrologers and in calculating holy days and festivals

And many of the Eastern Orthodox Christian churches still use the older Julian calendar, at least for calculating holy days.

By the way, many books now use 'CE' (for 'Common Era') instead of the distinctly Christian AD (Anno Domini, 'in the year of Our Lord')

I don't find it offensive. I was just wondering if lets say Jesus had never been born and everything, what year would it be in the USA?

Also to all the other countries that use different calendars, how do their computers and atms and such things work together? I mean if they used windows computers does it goes to whatever calendar that country has or does it goes to 2009?

I think for 99% of the world and computers, the year is now 2009, including in China and Israel. The Arabs may not like this, but can be comforted that almost all the world also uses Arabic numbers. It spread around the world as European Christians colonized the Americas, Africa, Australia, and south Asia