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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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  • leafers ultimate style avatar for user Vishwam Chand
    But Jesus was born on December 25th, wasn't he? So why is Sal saying he was born on New Year?
    (3 votes)
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    • old spice man green style avatar for user John Nolen
      Actually Sal stipulated the "theoretical birth of Christ" as January 1, AD 1. Christmas (or Christ's Mass) was declared by the Roman Catholic church to be celebrated on December 25. December 25 coincides with the pagan celebration of Yule, a holiday commemorating the winter solstice. Declaring Christmas to be on December 25, the church effectively co-opted Yule as a celebration of Christ's advent.
      (6 votes)
  • male robot hal style avatar for user Alex
    What about other calendars in use? Don't the Chinese and Hebrew schemes differ?
    (7 votes)
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    • piceratops ultimate style avatar for user Brett Nemecek
      The Chinese and Hebrew traditional calendars are radically different than the common calendar used by Western society. Both are largely lunar calendars (based on the phases of the moon, rather than on the position of the Earth in its trip around the sun, or the seasons), with some Solar elements built in. The Chinese calendar starts it's numbering with the foundation of one of its imperial dynasties. The Hebrew counts from the Biblical account of the beginning of the world (as counted, largely, by age of the Patriarchs).
      (5 votes)
  • winston baby style avatar for user smart180
    Why can't you just say for Plato's birth, the year -428 instead of 428 B.C/B.C.E
    (1 vote)
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  • blobby green style avatar for user Brenda Bain
    I was taught that the lack of year 0 was because the counting system of Roman numerals did not contain a number to indicate zero. Is this correct?
    (4 votes)
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  • leafers ultimate style avatar for user Mao, Nicco
    wasn't Jesus born on christmas
    (1 vote)
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    • piceratops ultimate style avatar for user AegonTargaryen
      No, that is a common misconception. Jesus was born sometime in the spring. His birth date was altered so that the early christians could snuff out the popular pagan holiday of Saturnalia. Saturnalia was a 4-7 festible thanking the god Saturn (Roman god of agriculture) for a good harvest before the winter. Saturn being a false god to the christians, they wanted to destroy that holiday.
      (6 votes)
  • blobby green style avatar for user Valentin Dascalita
    Still unclear about this:
    Lets just pretend JC was born on 1'st of January.
    On his first year birthday (1'st of January year 1) he would be 365 days old.
    A year before JC was born (1'st of January year (-1)) would count 365 days as well.
    So the difference between (1'st of January (-1) and 1'st of January 1) is 730 days (which means 2 years)
    Please correct me if wrong.
    *I still remember reaching this topic sometime in high-school with the math professor. She was for a year 0.
    Thank you!
    (1 vote)
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  • male robot hal style avatar for user Skeptic
    How long after Christ's death did we start using this system? Or did say people start suddenly writing the date as April 4th, Year 1 after he was born?
    (1 vote)
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  • piceratops ultimate style avatar for user nobody
    thanks for the correction, but I still don't understand why it is 1491AD
    (1 vote)
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    • piceratops ultimate style avatar for user Brett Nemecek
      The year is still 1492 AD, but that means it is 1491 years after the (theoretical) birth of Christ. It's the same with how Decades, Centuries, and Millennia work. The first year of a new decade is not on the 10, a century doesn't start on 100, and a millennium does not start at 1000. If you count from 1 to 10, that is 10 digits. To get to the next set of 10, you again start with 1, plus the original 10. Thus a new decade starts with 11, the previous 10 years belong to the first decade.
      (4 votes)
  • starky ultimate style avatar for user Raghav Arun
    Does that mean that all of our calendars are wrong, and we're a year before what they say?
    (1 vote)
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    • duskpin ultimate style avatar for user Quivico
      No because our system is entirely relative. This would only be right if a calendar system with a year 0 wasn't relative (the "true" time) and we based the calendar system we currently used on that one, but seeing as we currently do not have a calendar system which is the "true" time, we cannot say that we are actually a year before what another calendar system says. If we did have a year 0, then the current year would be 2016, but for the purposes of the universe, it would be like our current 2015.
      (3 votes)
  • spunky sam blue style avatar for user ved  prakash
    Do one year of one BC means one equivalent year of AD?Many ancient society
    have their calendar based on moon.
    (1 vote)
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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.