Why should anyone else care whether North Korea sets its own calendar system? The idea that any other country would "allow" or disallow an independent country to set its own policies is strange, unless those policies infringe on basic human rights. We don't have to agree with the new calendar, but we should mind our own business.
Although it's a little bit silly for North Korea to set a different calendar than the rest of the world, they can be backwards and out of sync with the world if they want. What are we honestly going to do to stop them? They can call their days whatever they want.
North Korea can do whatever it wants with a calendar system so long as the regime doesn't decide to launch missiles at its neighbors. North Korea's calendars won't change the civil calendar in place for the rest of the world. Religions already have their own lunar or solar calendars, so this shouldn't be that big of a deal.
The "Hermit Nation" of North Korea has made headlines for many insane reasons, from their saber-rattling nuclear tactics to the antics of their "Supreme Leaders." Now, they are in the news again for deciding to make their own calendar system. They shouldn't be allowed to do so - we already have a calendar system that's well-established and North Korea should stick to it.