One of the big hang ups I see in this discussion is that people are using different starting points. This is easy to do because time can be viewed as both relative and arbitrary as opposed to to solid and tangible.

Odds are we all use the Gregorian calendar to plan our lives around. Could be the work day, anniversaries, vacations, etc. Who created that calendar is fairly irrelevant, what is relevant is if there is a year zero and if a year zero is logical in any way.

Zero implies nothing, the lack of existence of said thing, in this case it’s a measure of time. That is why the Gregorian calendar has no zero year because zero implies there is no year, thus it becomes a point from which time can be measured forwards and backwards. Working from that point forward, time was considered to be In The First Year of the Common Era. “In the” denotes a lack of completeness to that year, once that year is complete it moves into the second year until that year is comeplete and so on. Similarly, there is no Zero Century, the time from when the 1st Century was deemed to start until the completion of the hundredth year IS the 1st Century and the pattern continues until now.

There are two further points of confusion that come to mind.

First is the way age is commonly counted which has been brought up multiple times. A person, place, thing, event, is said to have reached Anniversary X at the completion of said year, and this is fine. It would also be accurate to say that whatever is being commemorated is in the year X in the proceeding year because that year is in progress. It’s the reason the NHLs 100 year anniversary seemed to last two years, and teams do this as well on anniversary dates so they can milk those milestone badges all they can for an extra season of nostalgia.

When it comes to age it might be easiest to understand if it’s considered right from the birth. I have never heard someone refer to their child as being Zero years old. The child exists so a designation of non-existence doesn’t make sense, therefore the child is referred to as X-days, weeks, months old, and if the parent said their child was IN their first year it might be taken as somewhat ambiguous but also completely understood.

The second thing that most people get hung up on is the large round number at the end of a century or millennium and take it to mean a new one has begun, but do this tangible, small scale exercise. Assuming you have a full set of digits on your hands and feet start counting one set of them. If you’re counting your fingers/thumbs do you start at zero or one? Once you’ve finished counting them are you at 9 or 10? Odds are you start at 1 and end at 10, and even though the final finger is counted as a double digit as opposed to a single one numerically, we have no problem understanding that as a complete set. Continue with your toes. Do you start at 10 or 11? Do you finish on 19 or 20? Even though you’ve counted into the 20s from the teens it’s easy to understand how this is a full set and an appropriate way to measure and group our fingers and toes.

The same logic applies to years, decades, centuries, millennia, etc. We are IN the 2021st year since the Gregorian calendar began what is known as the Common Era, and once this year is complete 2021 years will have fully passed since that pinpoint in time. It is relative and somewhat arbitrary and we could discuss the merits of whether this past point in time was logical to base the passage of years by, but it is the system that the great majority of individuals use to run their lives around. Whether or not it’s sensible to change the understanding of the passage of time to suit the views of the masses could also be another debate entirely and one that I’m not particularly interested in delving into.

Edit: one thing I forgot to mention it that most people look at calendar calculations similar to a standard mathematical formula like they would with temputure, money, +/- stats, where zero is absolutely a quantity that needs to be passed though to get to the other side no matter which direction the calculation is being made. This may be why it may seem foreign to understand that the Gregorian calendar doesn’t use a zero year in a similar way.