There is no zero, as a starting point, when counting years. The moment one leaves 1 BCE or 1 CE he is immediately across the axis in the year 1 on the other side. Again, the digit zero is not a factor.
This is easily seen when taking just a 10 year span across the axis.
|10|9|8|7|6|5|4|3|2|1*1|2|3|4|5|6|7|8|9|10|
With pointer on any number on one side and moving exactly ten, one-year moves to the other side, it will be seen that the sum from where one starts plus the number where one ends, or ten full years away, equals eleven and not ten, or always one more than the actual number years moved.
For example: 537 BCE to 1984 CE using dates will not give one the proper number of full years. 537 BCE is a partial year and 1984 CE is a partial year, which when added together will give one full year. Therefore one must add 537 plus 1984 and substract 1.
537 + 1984 = 2521 - 1 = 2520 full years between each date.
or
BCE + CE - 1 = full years between the two dates.