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Correction calendar notation

Correcting the time difference calculation by taking into account that there is no year 0. Created by Sal Khan.

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Video transcript

I told you in the last video on calendar notation that, regardless of whether you're using BC, AD or BCE and CE, that there is no year 0, that we had 1 BC. And then we had that theoretical birth of Jesus, and most historians don't think that he was born right exactly on January 1, 1 AD. But there's no year 0. Right after that, you go from December 31, 1 BC, to January 1, 1 AD. There's no year 0. And despite the fact that I emphasized that in the last video, I didn't take that into consideration when I calculated how many years there were between Plato's birth and Columbus discovering the new world. And the reason why I didn't take that into consideration is that the year 1492, whether you want to call it AD 1492, Anno Domini 1492. Whether you want to call it that, or whether you want to call it 1492 in the Common Era. It's not 1492 years since the theoretical birth of Jesus, which we know is not the actual birth. He was probably born before that. It is 1,491 years since the birth of Jesus. And to think about it this way, let's just assume-- I'll keep emphasizing, it's a theoretical date that we're talking-- or the theoretical event, this kind of birth of Jesus that our calendars revolve around. If we talk about January 1, let's think about it this way. So January 1, 1 in the Common Era. How long is that since the birth of Jesus? It's not one year. You wouldn't just look at this and say it's been one year, because this is theoretically the day that he was born. So this is zero years, or almost zero years since that theoretical birth of Jesus. Another way to think about it is how long after January 1, the year 1 before the Common Era-- and I could've called this AD, and I could've called this BC-- what's the time difference between these two dates? So the way I calculated it before, I said, oh, this is one year after that theoretical birth. That's wrong. This is during that theoretical birth. But if I did it the way I did in the last video, I would have said, oh, that's one year after, one year before. You add them together, and you would get two. But that's wrong because there is no year 0. So January 1, 1 AD, or 1 in the Common Era is right over here. And then January 1, 1 BCE, is exactly one year before that. So there's only one year, one year difference. And the reason why the math is strange is because there is no year 0. If there was a year 0, then my calculation in the last video was correct. So really, the way that you would calculate the time between Plato's birth at 428 BC and Columbus sailing across the Atlantic in 1492, you would say, OK. This is 428 years before that theoretical birth of Christ. But this isn't 1,492 years after that theoretical birth. This is 1492 minus 1. So what you would do is you would add these two numbers. This is 428 before. This is 1,492 minus 1 years after. So you would add them and then subtract 1. So the correct answer-- so this is the correction part-- it isn't 1,920 years between Plato's birth and Columbus. We want to subtract 1 from that. It is 1,920 minus 1 years. So that is 1,919 years. The same way that the difference between 1 AD and 1 BCE, you could almost view it as positive and negative numbers. You say, oh, this is positive 1 minus negative 1. That would give me two. But there is no 0, so you would need to subtract another 1. So this is exactly 1 year difference. So just want to clarify that. That's what that no year 0 does to us.