It happened in 2000 when everyone celebrated the New Millennium and new century one year too soon. It happens every year ending in zero when everyone celebrates new decades too soon.
The way we construct our Western calendar [which is a convoluted story in itself] is different from the way we count our age in the West. They are inconsistent.
Birthdays are anniversaries of our births. Unlike the Chinese who assign someone the age of one when they are born... in the first year... we say someone is one year old when they have completed their first year.
Calendars, on the other hand, start counting in the same way that the Chinese count birthdays... in the first year, in the second year... in the tenth year. At the completion of the tenth year, the old decade is complete and the new decade begins. Hence, January 2010 on the calendar marks the beginning of the tenth year of the decade.
When Arthur C. Clarke wrote 2001, he used that year intentionally because it marked the beginning of a new millennium and, in his story, a new chapter for mankind.So, when we reach midnight on December 31, 2010, we will officially enter the new decade... 2011. But don't be concerned if everyone uses the convenient convention of celebrating a new decade today. After all, it is ten years since January 1, 2000 when we celebrated the Millennium one year early.
Any ten year span is a decade. Just because we can't agree when to celebrate it doesn't have much importance... except to organizers of celebrations. There are so many more other important things upon which we cannot agree.