Where would you restart the calendar?

[Anomalocaris]

I like Becc's beginning of the Renaissance though that's hard to date. You could pin it to Erasmus I suppose. Other really important sea change dates occur to me: Newton's death or birth or perhaps publication of Philosophiæ Naturalis Principia Mathematica; the publication date of Einstein's Theory of Relativity is another possible (I see Faith No More just beat me to this one); the dropping of the first atomic bomb; the inception of the internet.

Relativity, quantum mechanics, space travel, atomic energy, internet are all fine mile stones.

But principia set out the feasibility of the road and laid the first paving stone. So Principia wins.

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Why would you start the calendar year in the year Galileo/Copernicus was born? Why not the year they made their respective discoveries?

Doesn't matter. But others seem to prefer to set origin of the calendar to the birth or death of the actor rather than the date of the action.