But look at your fascinating chart! Besides there are only 12 months in a year. How many years are there? It only makes sense to use integers to count them, especially when dealing with negative and positive numbers. Also from looking at the chart there is no precise point. When dealing with this approximation, one uses numbers divisible by 10 or in this case 100 with 0 as a dividing point with signed numbers. To my eye 0 is a reasonable approximate maximum. You could say it was sustained until about 100 AD.