Like it’s supposed to have started when “Christ” was born, but why did people suddenly start counting them? Most people in the world would have been totally unaware of Jesus if he did exist, and it took a really long time for the general consensus of him being holy to actually gain traction. And when it comes down to it, why would he have triggered people counting years?
There are some previous answers that will hopefully help answer this question:
How confident are we that the year is actually and exactly 2016? Is it possible that at some point in the last 2000 years there were any significant timekeeping mistakes? and Did people in the Middle Ages call the year "one thousand and one" or "ten oh one"? by sunagainstgold
At what point did society begin referring to the year in modern terms (2012 etc)? by /u/Algernon_Asimov
Was the millennium from 999-1000 ad marked in any special way? by /u/savvysioux
Did people in 999 AD celebrate the new millennium? Were there any doomsday predictions similar to Y2K? by /u/haimoofauxerre
And a couple by me:
Did the Eastern Roman Empire date by Anno Domini or by Ab Urbe Condita?
But the calendar in which the year is 2021 is certainly not the only calendar. Within the past few weeks, it just turned 5782 on the Hebrew calendar and 7530 on the Byzantine calendar (based on calculations of the date of the creation of the world), and 1443 on the Islamic calendar.