Most of the controversy as to whether the third millennium begins on January 1, 2000, or January 1, 2001, is based on relatively straightforward arguments. One group believes that the change from the 1900s to the 2000s determines the appropriate date for the beginning of the third millennium.The other group argues that the numbering of years began with A.D. 1 and adding 2000 years to January 1, makes January 1, 2001, the appropriate beginning of the new millennium. The latter group points to the absence of a year zero (1 B.C. is followed by A.D. 1) as proof that 2001 is the correct year.The controversy is easily resolved by reviewing the way in which the present calendar was established (according to the 15th (1976) edition of the Encyclopaedia Britannicd).
There are various religious factors that delayed the development of a reliable Christian calendar. The Eastern and Western Christian churches began to diverge (disagree) almost from the onset of the Christian era. One area of contention was that the Eastern church would calculate the date for Easter each year and pass it on to the Western church. But the method of calculation was jealously guarded.