The confusion all stems from the fact that the calendar we use was devised before the numeral 0 came into use in the western world. The first year of the A.D. or C.E. should have been year 0, but it isn't. It's year 1.
The calendar goes,
2 B.C.E.
1 B.C.E.
1 C.E.
2 C.E.
Year zero is missing.
January 1st, 1 C.E. was the first day of the first decade of the first century.
The second decade started on 11 C.E., the third on 21 C.E. and so on.
All decades begin on a multiple of ten plus 1.
This decade extended from Jan 1st, 2001 to Dec 31st, 2010
The new decade begins on Jan 1st, 2011.
This from wikipedia:
The absence of a year 0 leads to some confusion concerning the boundaries of longer decimal intervals, such as decades and centuries. For example, each decade begins with a year ending in 1, not 0. The third millennium of the Gregorian calendar began on 1 January 2001, rather than the widely celebrated 1 January 2000. Likewise, the 20th century began on 1 January 1901.
This rule results from the fact that the Gregorian calendar begins with a year 1 instead of 0. Cardinal and ordinal numbering of years is therefore identical: The year 10 is the tenth year of the calendar and the end of the first decade. The year 11 is the first year of the second decade, and so on. In spite of this rule, years ending in 0, rather than 1, are commonly perceived as marking the beginning of a new decade, century, or millennium.
If the Gregorian calendar had begun with a year 0 as its first year, then the year 10 would have been the 11th year of the calendar and the first year of the second decade. Similarly, the year 2000 would have been the 2,001st year of the calendar, therefore the actual first year of the 21st century and the third millennium.