There are a variety of different traditional calendars from different cultures, but I strongly suspect business people in, say, Saudi Arabia, Dubai, or UAE use the same calendar we do. I in fact strongly suspect _everyone_ in those countries do, for their day-to-day non-religious date-related tasks.

Wikipedia confirms "The Islamic calendar is now used primarily for religious purposes, and for official dating of public events and documents in Muslim countries". http://en.wikipedia.org/wiki/Islamic_calendar#Uses