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.
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.
Korea kinda takes this to the extreme with birthdays.
The year 1AD wasn’t called 1AD in 1AD. The system was invented hundreds of years later.