Place value

A decade is ten years.

The first decade of the common reckoning consisted of years 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10. The first year of the second decade was 11, not 10.

The last decade of the first century consisted of years 91, 92, 93, 94, 95, 96, 97, 98, 99, and 100. The first year of the second century was 101, not 100.

The last decade of the first millennium consisted of years 991, 992, 993, 994, 995, 996, 997, 998, 999, and 1000. The first year of the second millennium was 1001, not 1000.

The last year of the twentieth century was 2000, which is why the year starts with "20--", and ends, like "10", in "-0".

Conseqently, 2010 is the last year of the first decade of the century, not the first year of the second decade.

Happy New Year!