This is a dilemma for many folks in doing planning for church historical events. When it comes right down to it, from the point of view of the age of, say, a congregation, it probably doesn’t matter if you celebrate the advent or completion of it’s 100th year.
But it is an interesting conundrum for others when determining decades, centuries and millennia as to whether to use one’s instincts (if they run that way; mine do not but then I’m peculiar) and say that the year 2010 was the start of a new decade and what went before was the first decade of a new century, or to follow the logic of the Heretic archivist and see that 2010 is the last year of the decade and therefore next Dec. 31/Jan. 1 is when we should look back.
I guess that the fact that so many people have trouble grasping this one is partly a “right-brain, left-brain” thing, but is not helped by the fact that there have been several acknowledged errors in the creation of the various calendars over the year and the inclusion of the erroneous (isn’t that term ironic?) statement that “a year zero was accidentally left out” among them seems to be readily accepted. The argument on the Heretic archivist site is, however, basically correct if you have a wish to check it out.