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How does one know how many years before year 0000(current years) say "300 BC" was?

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    Years are supposed to go xBC ... 1BC AD1 ... ADx. There is no 0BC.
    – Lawrence
    May 4, 2021 at 3:56
  • That depends on how you look at it. On one hand, you might say it was 4.543 billion years ago since that's how old the Earth is and the human measure of time "year" is standardized by how long it takes for the Earth to travel one time around the sun. On the other hand, the universe is 13.8 billion years old, so you might say year zero was 13.8 billion years ago, but then you get into the sticky wicket of how time is relative and time dilation at the beginning of the universe that would have made what we on Earth now consider but just moments last eons if you were there then. May 4, 2021 at 4:22
  • The real question is which calendar? For example, the astronomical calendar has a year zero. May 4, 2021 at 4:26
  • ...the fact the Romans in the 9th century started using their estimation of Jesus Christ's birthyear as the central reference point for counting years in the Julian Calendar but hadn't really gotten on board with zero being a number yet notwithstanding. May 4, 2021 at 4:27

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There was no year 0. The structure of the current calendar goes directly from 1 BC to 1 AD (or AD 1, as you prefer). AD 1 translates as "the first year of our Lord"; the folks coming up with the calendar back then didn't have a proper concept of zero (zero as a distinct number didn't reach Europe until it was far too late for inclusion in the calendar).

The lack of a year 0 is why pedants can point out that the third millennium started in the year 2001, not 2000. Years 1-1000 were the first millennium, 1001-2000 the second, and 2001-3000 the third; if we'd actually had a year 0, this wouldn't have been a problem, as the millennia would go 1st: 0-999, 2nd: 1000-1999, 3rd: 2000-2999.

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