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, the majority of the world is preparing to