The End of the 20th Century Next month we will celebrate the end of the 20th century and the beginning of a new millennium. The year 2000. Although the third millennium doesn't officially begin until Jan. 1, 2001, when every numeral in 1999 rolls over and the year 2000 comes in, a lot is going to be going on. When the sixth-century monk Dionysius invented the calendar, he was working before the invention of the zero, and therefore started his system with the year 1 AD, his calculation of the year of the birth of Jesus. Thus, the millennium doesn't officially end until the last day of the year 2000. However, t