Did the year zero never exist ?

Please explain my confused me like I'm 5 (0r 4 or 6).

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It depends which calendar you use! Every calendar picks a basically arbitrary system to uniquely identify each year, and in some of them "year 0" doesn't refer to any year.
The Gregorian, for example, goes directly from 1 BC to 1 AD, since 1 BC is "the first year before Christ" and 1 AD is "the first in the years of our lord." This doesn't make much mathematical sense, but it's not like there was a year that didn't happen--they just called one year 1 BC, and the next year 1 AD.
ISO 8601 is based on the Gregorian calendar, but it includes a year 0. 1 BC is the same year as +0000; thus 2 BC is -0001, and all earlier years are likewise offset by 1 between the two calendars.
- If ISO says there was a year 0, there was. There’s only one thing better than perfect : standardized !
  
  I need my standardized fixed calendar now dammit

Yes. They skipped right over. It confused many people at the time: a whole year of their lives, gone. Many centuries later when zero was invented, an explanation was finally offered as to why that happened.
- 😂
  
  Serious answer about what the year would have been in 1 AD, according to 63-year-old Emporer Augustus: ~~DCCLIV~~ 754 Ab Urbe Condita
  That means "from the founding of the city" - they based their calendar on the mythical founding of Rome, as calculated by Verro, who himself was not long dead at that point. Before that, they just counted the years of each person's reign Japanese-style. Probably other people in the ancient world had older calendars.
- For those who don’t know, the concept of zero wasn’t discovered until the sixth century in India, and then either transferred or rediscovered in about the eighth century in Arabia.

When you consider the time as a number line, years are not points at integers (which would in some way warrant a year 0), but rather periods between them. Year 1 is the period between 0 and 1, and before that was -1 to 0, or year -1. There is no year 0, because there isn't anything between 0 and 0
- That makes sense, but trying to square that off with the idea that the year 2000 is the start of the 21st century is hurting my head.
  If year 1 is the 1st year, then surely the first year of the 21st century should be 2001?
  
  https://en.m.wikipedia.org/wiki/Millennium#Debate_over_millennium_celebrations
  
  If year 1 is the 1st year, then surely the first year of the 21st century should be 2001?
  It is. The system is confusing.
  
  It should all be zero indexed. Positional number systems like we write with are (600=0600) but our language isn't, which causes this problem. Basically, if 2004 is the 20th century the gospels took place in the 0th.
  
  the idea that the year 2000 is the start of the 21st century is hurting my head.
  That may be because it is not. The first century was years 1 to 100. The second was 101 to 200. The 21st is therefore 2001 to 2100.
  What you're probably referring to is the "cultural century" which was considered to have started when the lead digit changed from 1 to 2. The same thing happened quite recently when some people argued 2020 was the start of a new decade (again, it wasn't)
- This explanation is unclear to me. Why do we choose the later of the two endpoints of the year for (0, 1) but the earlier of the two for (-1, 0)?
  
  The language is rooted in the same logic as people. Your first year was between the ages of 0 and 1. The first year before you were born is between -1 and 0. There is no 0th year because 0 is a point in time and not a range in time.
  
  Because until the Middle Ages, Europeans were afraid of the number 0.
  
  Absolute value. Both systems count time from the same epoch, or zero point.
  One year before the epoch is 1 January 1BCE One year after the epoch is 31 December, 1CE.
  Half a year before the epoch (-0.5 years) is June 30, 1BCE. Half a year after the epoch (0.5 years) is July 1st, 1CE. These dates occur within the first year before the epoch, and the first year after the epoch, respectively.
  
  Remember originally that -1 was 1 BC, meaning 1 year before the birth of christ. The negative numbers are measuring the distance away from 0.
  Edit: in the positive direction, the 1 was 1 AD, meaning the first year of our lord. Just like when talking about the reign of kings/queens, the first year of their reign is 1 and the 14th year that they reigned is 14. I believe the timekeeping for Ages in LOTR may also be similar.
  
  Moving from too... not too from
  
  For the same reason why 1.5 is on the right from 1 but -1.5 is on the left from -1
- If we were starting from scratch, it would probably be better to go with two year zeroes, so it would fit normally into positional number systems, and then you could even talk about 0.5AD for the relevant summer.
  Unfortunately, positional numbering wouldn't be invented in the old world until hundreds of years after the Christian calendar.
  
  The only positional numbering system I use daily (base 10) has only one zero. What system are you talking about?
  
  So in your idea there would be year +0 and year -0 before it, right?

Years exist. We decide what to call them. You and I agree to call this year 2024, but that's only an agreement. Some people call this year 5784.
We call the system we use "The Gregorian Calendar", because of a Pope named Gregory. That system is mostly the same as "The Julian Calendar", with some important changes to make the calendar match the changing of the seasons better. In the Julian calendar, they decided to count the years starting from when they thought Jesus was born. They chose his birth year to be "The first year of our Lord". We call that "year 1" for short.
The people who created that system (the Julian Calendar) didn't understand 0. The year before "The first year of our Lord" was called "The first year before the birth of Christ". We now call these "AD 1" ("anno domini", because Latin) and "1 BC" ("before Christ"). Since they didn't understand 0, they didn't call any year "0". We have kept the tradition, because reasons.
Some other systems have relabeled the year before "AD 1" as year 0, but that's not how the Gregorian Calendar works, and that's the calendar that you and I have been taught to use.
- "They," i.e. the catholic church, or whoever was tasked with coming up with a calendar, absolutely understood the concept of zero in the 1500's. Yes, Zero took a bit longer to formalize and enter the zeitgeist of the public consciousness, but this myth of zero being some kind of unknowable thing for thousands of millennia is naive.
  I'd go so far as to say that a year zero in a calendar is useless. There should be a starting point of course, but calling it yero zero instead of year 1 is dumb.
  
  By that part, I was referring to the people establishing the Julian Calendar, not the Gregorian. I've edited my comment to clarify that.

https://en.wikipedia.org/wiki/Year_zero
The year of Jesus’ supposed birth was counted as year 1 AD/CE. The year before that is considered year 1 BC/BCE. It’s worth noting that the concept of zero didn’t yet exist back then. https://en.wikipedia.org/wiki/0#History
- Probably worth noting that the Gregorian Calendar was an invention of the 16th Century. It was invented to deal with the problems of the Julian Calendar and the creators would have had a firm understanding of the concept of zero. The AD/BC split was all about the assumed year of the birth of Jesus of Nazareth (according to Christian mythology). The year of his birth was set as the first year Anno Domini or "The year of the Lord". Or the first year where Jesus was kicking about. The year prior to that would therefore be the first year before "Before Christ" was alive, and therefore the year 1 BC.
  
  Especially weird considering that Christmas has been set to December for a long time, so 98% of year 1 AD was actually before the ostensible birth of Christ (I know that scholars now think he was born in April or something, but they probably didn’t always)

ELI5 answer?
In the conventional calendar, there wasn’t a year zero and it wasn’t skipped. Zero is the moment in time that we use to begin counting time.
Think of an elementary school style number line: …-3_-2_-1_0_1_2_3… Each number is one year apart. This makes the numbers measure something like Age. If you are 3 years old, you can count 3 years between 0 and 3.
But a year is not an Age. It is the span of time between ages, and the years we name are actually the spaces between the numbers on the number line. So the first year (1 AD/CE) is the first space after zero (between 0 and 1), and the first negative year (1 BC/BCE) is the first space before the 0 (between -1 and 0).
Then there is the astronomical calendar, which does have a year zero. They get this by naming the year (the space on the number line) after the number to the right side of the space on the number line.
- By this description, year zero is the time between the 0 and the 1 for the same reason the time between 10 and 11 is the year 10.
  
  Your logic is correct, but you are off by 1.
  0-1 is year 1
  10-11 is year 11

i hope all these conflicting answers in the comments have made you less confused, OP.
- 😁

The anno domini (AD) dating system started in 525. The concept of zero did not make it to Europe until the 11th century.

Years are ordinal numbers, the kind of number that tells you which place you finished in a race, and as such cannot have zeroes or negatives. You're living in the 2,024th year since the instant that began the Common Era. "0th" and "-1st" are not valid expressions for years for the same reason that you can't place 0th in the Olympics
- Programmer clutching their keyboards screaming "Arrays start at 0!"
  
  Ordinals are largely used for counting and when you're counting you kind of do start a zero, most people just don't say it. When you count 1... 2... 3... it would work just as well to start 0... 1... 2... 3... So programmers can rest easy.

Nope. 1 BC/BCE -> 1 AD/CE. That's just how it was designed.
- The concept of zero had not even been discovered at the time the calendar was designed.
  
  Almost. The first recorded use of 'Zero' was in Mesopotamia in 3BC/BCE. https://www.history.com/news/who-invented-the-zero
  
  this is absolutely wrong.

When someone decides to change the way that they keep track of time, the new calendar typically starts at 1, as in "the first year of this new era". It's not that there was no existing year before that, just that it doesn't make sense to start as zero.
It's not like the Gregorian calendar that we use now existed in -1 and then rolled over to 0 and then 1. They just started the new one at 1, and for a period of time, there was surely some overlap in people using both calendars, until one was phased out entirely.
- The year 1AD wasn't called 1AD in 1AD. The system was invented hundreds of years later.
- Korea kinda takes this to the extreme with birthdays.

Arbitrary decision is arbitrary.

They left out 0 from the list. It basically jumps from -1 to 1.

The switch to the current system of using the theoretical birth of Jesus as the start of our calendar occurred in the 6th century, 500 years after the fact. They picked a year based on what evidence they had for when the birth of Jesus occurred with a margin of error of about ~30 years.
When this occurred and we started observing years in Anno Domini, whatever local calendar was being used was immediately replaced by the year 525, and retroactively everything before that was assigned it's proper year. This ends with AD 1 and directly starts with BC 1 going the other direction. No year 0 was observed in this switch.
- Also note that before this switch, years were often designated in relation to the founding of a city or by the start of a ruler's reign. There were always ordinal numbers, so the first year of a reign would be year 1, and there was never a zero, because it was year X of a previous reign.

They switched when years are counted. BC years are counted at the beginning of the year. AD years are counted at the end of the year.
The halfway mark in the first inch of a ruler is 0.5". The first inch ends at 1.00. 1.25" falls in the second inch of the ruler.
We recently completed the 3rd month in the 2024th year AD. We have not completed the full, 2024th year yet; but we are in it. This most recent April 1st was 2023.25.
1-Jan-1 BC was almost a year before 31-Dec-1 BC. 31-Dec-1 AD was a year after 31-Dec-1 BC.

No, in our calendar system there was year 1 BC followed by year 1 AD. So no zero. It’s just how they set it up, they’re human made ideas anyway. Many countries do not even use this system, for example it is currently year 2567 in Thailand and year 113 in North Korea.

like I'm 5 (0r 4 or 6).
Okay then.
Before dawn of technology advancements that we have today, people did stuff in a very different manner, for the sake of this explanation, I will call it "primitive"
As brilliant as human beings are, they often forget little things (little because may not have higher priority at that particular time) and dates is one of them.
Even now, if you happen to forget today's date, and do not have means for referring that (like looking at your smartphone or watch, some digital billboards and whatnot),
what you would naturally do is refer back/forward, to the closest (recent/upcoming) date and day where a memorable event occurred/will occur. Events like your cousin's birthday, trump impeachment, the coming football derby or the coming elections date. then you start counting with your fingers towards/backwards to the current day. This is "primitive"
These variations of calendars that currently exist today have their own sort of "memorable event".
The most widely used today is AFTER CHRIST (AD). (Of which, to go back past that, they should have used count backwards tactic, i.e. -1, -2, -3, -4; Eg: -4AD; but instead, -4AD becomes 4BC which is BEFORE CHRIST. That is why counting forwards in BC, number decreases 😏 )
To answer your question;
"Year zero" is the year where that particular memorable event occurred.
But as I demonstrated above, we use that year as a reference to count forward/backwards the following/past years.

Can i state the obvious reflect on if this question even makes sense?
We are currently the 2024 year since we began counting and probably didn't do so from day 1. Instead we took a significant cultural event and marked it the beginning. Adapting any initial time keeping to it.
We centered the beginning of this count on the life of someone who we cant proof ever existed. Great start.
we have likely been tracking sun cycles from much before but we cant exactly call our time keeping records reliable scientific measurements. Different civilizations and cultures had different ideas, may dispute data and eventually all had to make way for the teaching of the church.
There is no year 0, the calendar is a construct of time But doesn’t keep nor measure it.
- One nitpick, Jesus was almost certainly a real figure. There are many records indicating someone with that name was in the area at the time, and that they were executed by crucifixion.
  The religious stuff, obviously no way to prove. But as a person, the historical consensus is they existed.
  
  One nitpick, Jesus was almost certainly a real figure. There are many records indicating someone with that name was in the area at the time, and that they were executed by crucifixion.
  No there isn't. There's tons of people who've claimed they've found records but ultimately none of them can be produced or are based on other accounts like Josephus who doesn't ever directly reference Jesus. Further none of his original writing survived. Only reproductions, and the earliest one is from 11th century. Or Tacitus who was born after Jesus was dead. So no direct knowledge or evidence of Jesus as a individual, just a second hand accounting at best. Oh and also, no originals exist. Just copies dated back to the 11th century...
  All "evidence" only starts 1000 years after Jesus actually lived... supposedly written by people who were born after Jesus died... and would have written that stuff 50-100 years after his death.
  There is no actual archaeological evidence that "Jesus" existed. And a mere 3 references that exist outside of the bible that I'm aware of. All of which are not original manuscripts.
  Edit: All of this to say, there is no consensus... and to claim there is consensus on the matter is a christian/catholic claim. Not an actual historical consensus.

Zero doesn't even exist so how would year zero.

I’m no expert but I assume that the year Christ died would be “year zero” (assuming you’re talking about anno Domini (AD) and before Christ (BC)) since we started counting after that.
EDIT: reading more on the topic I might be completely incorrect with my above statement. If someone else knows, please do correct me
EDIT 2: I found this on Wikipedia which talks about a “year zero”

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