I'm just grateful that one calendar became the international standard, otherwise the combinatorial explosion would have been even worse. Imagine having to track not just one or two local calendars plus one international one, but the half-dozen different calendars of each of the countries you do business with. "Let's see, the French calendar has their new year's date on Charlemagne's birthday, the German one dates their new year on Bismarck's birthday, Switzerland follows the Gregorian calendar, but Luxembourg never signed onto the Gregorian calendar and they follow the Julian calendar so they're 11 days behind. No, wait, it's up to 13 days behind by now... or was it 14? Darn, I gotta go check again."

I'm just really glad that that's not what happened, and that we ended up with just one international calendar. So that even in countries that don't use it for religious holidays, there's still only two or three (usually two) calendars to track. Which is confusing, but way less confusing than having to track dozens of them.