Well Arabic countries traditionally use the Islamic calendar (luna based) rather than the Gregorian calendar (solar based). If memory serves Myanmar (nee Burma) uses a different calendar as well.
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
What's your point? They can afford to switch from Islamic calendar to Gregorian, because the majority already use Gregorian. It doesn't mean that it will be easy to make a country switch to a 13-month calendar that nobody else use
