Crack Theory Sharing Time!
Whatever crazy thing they were doing that broke Detroit, actually broke Detroit and the rest of the Fallen World quite a bit. There always were 360 days in a year until there wasn't any more and never had been. Pouring a dose of mana into the dragon lines and fixing the resonance mended a tiny portion of the massive damage done, and now there were always 360 days in a year again because we've brought the cracks in the fallen world back together on that particular issue.

Few other people realise the cracks extend so far beyond Detroit.

Alternate Crack Theory!

The world is and has been broken as well as fallen for some time. Detroit is a fracture, a network of breaks and stress lines and thin material. It is a symptom of the world's damage, rather than being simply a portion of the world that has been damaged. Things that happen in detroit have the potential for massively more powerful reactions from the world because it is pulled tight here, tense and precarious.