Not only is our numbering system arbitrary, but it's also strictly Western. He was an astronomer from Scythia, in what's now southwestern Russia. In the year 525, Dennis was compiling a table of the dates for Easter. In the midst of his labors, Dennis concluded that the system for numbering years was shortsighted. In those days, people reckoned years in terms of the reign of kings and emperors.
Trouble is, sovereigns had a habit of dying.
Thus, no system lasted for many generations. But Christ's birthdate was fixed and unchanging and permanent. Dennis chose the year of Christ's birth as his numerical North Star. So he counted backward to the year A.D. 1, and that's how we got our system. Because Dennis lacked our research tools, we must cut him some slack.