First of all, I doubt that anybody in this group could really give a
hoot about what the calendar says - we just want to know when the party
is scheduled. The biggest party is scheduled for Dec. 31, 1999 with a
smaller party on Dec. 31, 2000.
There is however something within us that cries out "innumeracy!!!" when
we hear people refer to Jan. 1, 2000 as the start of a new millenium -
it seems intrinsically offensive to our mathematical sensibilities.
After all, there was no year zero. (This is not a matter of opinion or
philosophy, it is a simple fact that Dionysius set up a calendar which
went from 1 BC to 1 AD - check your history books and forget about your
ruler.) So Jan. 1, 1 AD marks zero time from the start of the first
century, Jan. 1, 2 AD marks one year into the first century etc. This
gives us centuries ending in 00 years and beginning in 01 years.
Similar rules apply to decades and millenia.
But, some would argue that aesthetically 9/0 is the natural place to
change decades/centuries/millenia rather than 1/0. All that is required
is to define the first decade as having only 9 years, the first century
as having only 99 years and the first millenia as having only 999 years.
This seems a small price to pay to be able to say that 1990 is in the
90's, not in the 80's.
Another line of reasoning is that Dionysius actually believed Jesus to
have been born in the year 1 BC, thus 1 BC actually behaves for
numbering purposes as a year zero (being the year in which the Lord was
born). Hence Jan 1, 1 AD is in fact one year into the AD era, but we
have the problem of saying that (for example) June of 1 BC is actually 6
months into the AD era!!! Also (left as an exerercise for the reader),
when are the transitions between centuries in the BC era? Do we really
want things to be so lopsided between AD and BC century transitions?
The final stage into which this discussion degenerates is "what year was
Jesus actually born in?". (Its generally considered to be not later
than 4 BC, which means we all missed the party a few years ago
anyways.) While the question is interesting for historical and
religious reasons, it is not going to change our calendar and hence
shouldn't affect when we consider the centuries to change.