It's weird how geological time works. Eras start and end bit by bit over the course of hundreds of thousands of years. That sounds like a big uncertainty but that sort of timescale, on which the climate can overhaul itself completely and entire species rise and fall, is instantaneous compared to the age of the earth. Any hypothetical sentient creature would have no idea it was living through a major turning point. The Silurian slid into the Devonian as land plants became A Thing and insects started to wonder if 'pilot' might be a good career path, but there was no one moment when one thing ended and another began. That's not how that works...
... except for the Cretaceous. The Cretaceous had a Last Day and it was probably in April, and then the next day it was the Paleogene.
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