Egyptians didn't have cranes, but that didn't stop them, they used what they had (alot of /slaves/ workers died but that's not the point), and made amaizing pyramid structures that stand tall to this day.
I mean, lemonade came from a dude who only had lemons on his farm (at least it's what I believe)
What ever life gave you, it has potential that might not have been discovered, yet. I mean, who'd thought a long-cross screw and nuts could be used to lift a tonne of vehicle for a tyre change?
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 occured/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).
To answer your question;
"Year zero" is the year where that particular memorable event occured.
But as I demonstrated above, we use that year as a reference to count forward the following years.
Heck, they would shove them in your dreams if they can (and they will, using neuralink lol); and then find crevices on the laws to navigate through, like crabs side-walking to move forward.
Woa WYA