Technically, you are correct, but from the start of 1 BC to the end of 1 AD the length of this period is 2 years. So counting backwards to the end of 1 BC we can consider that it is equivalent to the year -1 after moving backwards 365.25 days. Likewise counting forward to the end of 1 AD we can consider that it is equivalent to the year 1 after the passing forwards 365.25 365.25 days.
With this being true, we have a theoretical 0 point in time on a numbers line when we are describing the years when events occurred in history, but because the various calendars we use have different starting points along this number line with respect to time we have a transition point, from BC to AD which can be considered to be the point zero along the number line describing time which has no length as a period of time. It just happens to be the transition point from a counting system that goes backwards in time, that increases negatively in value, from when theoretically, Christ was born, and a count system that goes forwards in time, that increases positively in value, from when Christ was theoretically born.
To overcome this supposed dilemma I have set my starting point for mankind from when Adam was created as the point on the timeline that I use to calculate time, as the point zero on the numbers line that I use to do my calculations for time on. That way I avoid the transition from the BC and AD numbering system. I have described my reference time period as, "After Adam," (AA), and considered the starting point to be after God had finished creating Adam. Using this method, I can create a chronology of the scriptures, without worrying about the BC and AD timelines.
To determine what the BC or AD year timeframes should be, I simply subtract 4100 from the years calculated on my AA time line, to determine the year in the BC and AD system. If the number is negative, then the year calculated if representing the BC time period, and if the number is positive, it represents the AD time period.
Now, I can determine the year from when Adam was created, to the end of the seventh Age, when Christ comes a second time to Judge all the peoples of the earth to determine whether they are righteous or unrighteous which determines whether they die the second death or not.
I know that this method of determining the time line for my chronology of God's timeline is reasonably accurate, with respect to the AA timeline, but I also accept that when correlating of the AA timeline with the AD timeline, that this is fussy from the perspective that there is no definitive "correction" number that can be used to move from one timeline system to the other. There will always be some "error" in how they are related to each other in real time and the size of that error is dependent on the assumptions that I had to use to reach my determination of the time.
This was why Christ could state that no one knows the "real" hour or day when the Second Coming will be.
Quite a few assumption have been expressed in the calculation outcomes for the duration of this time period as to it s duration.
Shalom
PS: - I define time to be the length of a period, between two non events, i.e. the starting and finishing points which have zero time associated with their passing.