"he looked again at the road which lay as before yet more dark and darkening still where it ran on to the east and where there was no sun and no dawn and when he looked again toward the north the light was drawing away faster and that noon in which he’d woke was now become an alien dusk and now an alien dark"
A rare colour photograph of the Trinity test.
Public DomainA rare colour photograph of the Trinity test. - Credit: Los Alamos National Laboratory

As oblique an ending as McCarthy as written, but taking a rough estimate of the dates in the novel and Billy's location, it seems clear McCarthy is referring to the Trinity Nuclear Test, which took place on July 16, 1945, at Alamogordo, New Mexico.

Following the success of the Trinity test, two atomic bombs were prepared for use against Japan during World War II. The two bombs subsequently dropped on Hiroshima on August 6 and Nagasaki, on August 9, killed at least 148,000 people immediately and many more over time. By 1950, the death toll was over 340,000.