It is important to learn from past mistakes, although sometimes there are good reasons for mistakes recurring. The issue concerning the start of the millennium has spilled over to the issue of when the new decade starts. Indeed, at the end of 2009, there are reviews of the decade in the spheres of sport and human interest for instance. However, the end of the first decade does not coincide with the end of the year 2009. The new decade actually begins on January 1 2011.
The correct date for the turn of the decade seems strange to many persons—perhaps rightly so. The argument of those who saw the start of 2000 as the turn of the millennium and 2010 as the beginning of a new decade is that is that counting normally starts at zero. For example, when you first come into the world, you are zero years old. Therefore, when you are 40, you have already spent 40 years on earth. By that logic, the year 2010 indicates that 2,010 years passed since the birth of Christ. The argument is logical and sensible but uses an incorrect premise.
The premise for that argument is flawed because the calendar year is really an ordinal number. Ordinal numbers denote place, rank or position and are followed by st, rd, nd and th. For example 1st, 2nd , 3rd and 4th are all examples of ordinal numbers. Cardinal numbers are normally represented in pure form: 1, 2, 3, 4. Typically, when we write the date, we use cardinal numbers—particularly for the year. For example, the form January 1 2011 uses cardinal instead of ordinal numbers. When we use an ordinal number in the date, we still do not represent the year as the ordinal number it should be (January 1st 2011).
This gives some folks the impression that the year is a cardinal number when it is ordinal. The reality is that the calendar year began with Anno Domini 1 (AD 1). That translates to the 1st Year of Our Lord, although when Jesus was actually born is immaterial (as though December 25 is really the birthday anyway). Since this renders the calendar year an ordinal number, it suggests that the year we are in is not completed until the end of the year. This is unlike our age, where our age represents the quantity of years we have lived. So in 2010, that is really the 2010th year, which would be complete at the beginning of the 2011th year.
The main issue that makes 00:00:01 on January 1 2011 the start of the new decade is the fact that the calendar year is an ordinal number and began at AD 1. However, folks would likely measure the decade on the premise of cardinal numbers. This is because we refer to decades as the 60s, 80s and 90s etc.
It would not be sexy to consider the millenium as starting in 2001 or the new decade as 2011. For that reason, the mathematical reality would not stop folks from marking the new decade from January 1, 2010 instead of January 1, 2011. It is much easier and uncomplicated to ignore the ordinality of the calendar year. Instead of swimming against the tide, it is far easier to go with the convention that marked 2000 as the start of the new millenium and 2010 as the start of the new decade.