I understand that there are different calendars, and most definitley, Muslims follow the Islamic calander so they can have their holidays ar the right time, but Internationally, the christian calendar is used.
It's the same phenomenon as most people in international business speaking English.
If there is some international transaction, which dating system is used? The "common era" system.
The UN uses it, most, if not all, countries use it in international affairs and trade, if I'm not mistaken.
They celebrated the Millenium in Cairo, Tokyo, along the great wall of China, and all around the world.