The Julian calendar, which it replaced, computed the period of the earth's orbit around the sun at 365.25 days. Pope Gregory XIII's reform substitute… - Graham Hancock

In North America the evidence is that hunter-gatherers bounced back quite successfully within less than a millennium of the onset of the Younger Dryas, and thereafter there is a thin but fairly continuous archaeological record. What is mysterious is not so much the early appearance of mound-building in this new age — perhaps as early as 8,000 years ago, as we've seen — or the sophistication of sites such as Watson Brake 5,500 years ago, nor even their obvious astronomical and geometrical connections to later vast earthworks such as Moundville and Cahokia, but that in this early monumental architecture of the New World memes of geometry, astronomy, and solar alignments consistently appear that are also found in the early monumental architecture of the Old World at iconic sites such as Stonehenge and the Great Pyramid of Giza. A tremendous leap forward in agricultural know-how, coupled with the sudden uptake of eerily distinctive spiritual ideas concerning the afterlife journey of the soul, also often accompanies the architectural memes. It's therefore hard to avoid the impression that some kind of 'package' is involved here.

Although I have no doubt that the ancients had developed special means for manipulating the physical world - means that our own technological society is sometimes unable to match - I think it is obvious that their main project was a profound spiritual quest & the dilation of the mind through the pursuit of esoteric knowledge. It is in this area, I believe, that we have the most to learn from our ancestors, where their great minds were put to work for thousands of years to penetrate the riddles of existence.

[The cosmic impact that started the Younger Dryas] marked the end of their story, and the end of an epoch, really. There's not a single Clovis point found anywhere in North America that's above that black mat. They're all in it or below it. And there's not a single mammoth skeleton anywhere in North America that's above it. A huge part of the die-off could have been as a direct result of the impacts themselves, but impacts and airbursts south of the ice cap, particularly as far south as New Mexico, would also have set off wildfires. There's overwhelming evidence that gigantic wildfires raged at the onset of the Younger Dryas — in fact, more soot has been found at the Younger Dryas Boundary than at the Cretaceous-Paleogene Boundary. We did the calculations and it looks like as much as 25 percent of the edible biomass and around 9 percent of the total biomass of the planet was on fire and destroyed within days or weeks of the YDB. So in many areas if the animals weren't killed outright they wouldn't have been able to forage enough food afterwards to survive. The grass would have burned up, leaves on trees were gone. ... And you know, the other thing is that when comet fragments come in they're traveling incredibly fast and they literally punch a hole in the atmosphere. They actually push the air aside and they bring in that super cold from space, and when they explode in the air that cold plume continues to the ground and you literally have things frozen in place if they were close enough to where the plume came down. It's possible they were fried and then frozen all within a matter of seconds.