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.
Put your pencil on any number on one side and move exactly ten, one year moves to the other side. You will see the number from where you started and the number upon which you arrived add up to be 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 is a partial year and 1984 is a partial year, which when add 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.
BCE + CE - 1 = full years between the two dates.
If one is interested in computing the date on the other side of the axis when given only one date and the number of years moved. See the following link: BCE - CE or CE - BCE Find the date