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.

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.

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

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.

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

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.

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

Nope. 1 BC/BCE -> 1 AD/CE. That's just how it was designed.

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 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.

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

Arbitrary decision is arbitrary.

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.

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.