What about the Islamic calendar? If your software handles anything having to do with dates and is being used internationally then it seems to me you have to take that one into account: eventually somebody will want to see their calender or input a date in their local date handling system.
Edit: I am confused about the downvotes. I point out that he can't assume there is only one calendar system in use, even for general software.
I guess you're being downvoted because your point about Islamic calendars is (mostly) wrong. Islamic calendars (but I don't even live in a country where it's used, so it might be more nuanced than what I say) are used mostly for religious purposes; you 'can't' (practically speaking) use only the Islamic calendar because many things you care about (like, your barber's appointment) would be done in 'Gregorian calendar'. So an Islamic calendar is only needed in a very limited subset of all software packages, even in Islamic regions. It's not as simple as 'in some areas people use Gregorian, on others Islamic', which your post seems to suggest.
Iran is using the Islamic calender. Just look at their railways website http://www.rai.ir/index.aspx Or the website for the airport in Tehran ( http://ikia.airport.ir/ ), which uses persian dates for the weather forcase and gregorian for flight details.
And yet, when I am contacted by Iranian researchers, they all express dates and times in the Gregorian calendar. Obviously those with international contacts are not 'average', but still there is/must be wide spread use of the gregorian calendar - as your airport example confirms.
Either way, let's still for the sake of the argument say that all of Iran only uses the Islamic calendar. That still reduces the GP's point from 'to sell software in countries that are mostly Islamic, you must support multiple calendars' to 'to sell software to Iranian customers you must support multiple calendars'. Iran, just to state the obvious, is widely embargoed across the world, and it's quite challenging to sell anything there, legally (I have first hand experience to the extent that it wasn't worth the time for 5 figure contracts - and I suspect that even 6 figures wouldn't make it worth it).
To conclude, I don't think your contra-anecdotes are enough to counter the position that assuming Gregorian is 'good enough' in the vast majority of cases (again, those few writing software for Mormon record keeping, or history journal analysis tools, or -yes- software for Iranian state systems, probably know so much about dates and times that they don't need '100 things' lists to know what to look out for).
Edit: I am confused about the downvotes. I point out that he can't assume there is only one calendar system in use, even for general software.