This is a date that no organization, including NIST,
has the authority to regulate. However, one logical answer
to the question is that because there was never a year
"zero," and a century must have 100 years, then each century
must begin with a year numbered "1." In other words, the
20th century should be considered as ending on Dec. 31,
2000, and the 21st century as starting on Jan. 1, 2001.
However, human nature being what it is, most of us will
still opt to have that "once-in-a-century" New Year's Eve
bash on Dec. 31, 1999.