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).
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).