Huh. It seems there are those who consider the calendar to be contextual too.
Some writers like to point out that since the common calendar starts from the year 1, its first full decade contained the years from 1 to 10, the second decade from 11 to 20, and so on. The interval from the year 2001 to 2010 could thus be called the 201st decade, using ordinal numbers. However, contrary to practices in referencing centuries, ordinal references to decades are quite uncommon.
Following the link for ordinal numbers embedded above, we find this:
In linguistics, ordinal numbers are the words representing the rank of a number with respect to some order, in particular order or position (i.e. first, second, third, etc.). Its use may refer to size, importance, chronology, etc. They are adjectives.
They are different from the cardinal numbers (one, two, three, etc.) referring to the quantity.
Ordinal numbers are alternatively written in English with numerals and letter suffixes: 1st, 2nd or 2d, 3rd or 3d, 4th, 11th, 21st, 477th, etc. In some countries, written dates omit the suffix, although it is nevertheless pronounced. For example: 4 July 1776 (pronounced "the fourth of July ... "); July 4, 1776, ("July fourth ..."). When written out in full with "of", however, the suffix is retained: the 4th of July.
Since the modern western calendar system you do indeed use (the time/date function of the computer you are using to read this is based on it) is in actual fact an ordinal number record, and since that system in fact does not contain any such silliness as a "year 0" (if you were to cycle the format back far enough the transition would read 12/31/01bc - 01/01/01ad), it follows that the "common usage" mentioned in the first cite above is yet another example of human stupidity on public display.
Just this once, and do try to follow along, it goes something like this:
A decade consists of ten consecutive years.
A century consists of ten consecutive decades.
A millennium consists of ten consecutive centuries.
See the pattern here?
The first year of a decade is year one (1). Consequently, the last year of a decade is year ten (10).
Do keep up, there's more!
If you sequence ten decades in a row, the result is called a "century" and the convention is to identify that time period by it's final year number. Thus, once a century is completed it is written X (with x- being some number) 00; 200bc, for example, or 1700ad perhaps. This system does lead to the slightly confusing practice of referring to a given year, say 1776 (a year of some significance to American readers), as having occurred during the 18th century.
Continuing to follow the pattern we observed earlier, ten centuries transpiring one after the other is widely known as "a millennium". 1000ad would thus be ten (10) centuries (00). See?
Given all the foregoing to be true, the most previously completed millennium would have been the second following the transitional event in the calendar system we use in much of the modern world. And, indeed, the last year of the 20th century was in fact recorded as 2000ad. It would then consistently follow that the succeeding year would be the first in the new millennium/century/decade that follows on along from there, wouldn't it?
So, to re-cap: the millennium following after the second would be the third. It is scheduled to consist of ten centuries ending in the numbers 21 through 30. Each century is demarcated into segments of ten decadal sub-units which, in turn, consist of ten single year segments, numbered conveniently enough 1 through 10.
Now hover the mouse cursor over the time display on your monitor and read for yourself today's date. January 2, 2010 maybe (if not we have a problem of an entirely other dimension)? Now, the quirks of our cack-handed cousins across the pond notwithstanding, this is written as 01/02/2010 and reads: the second day of the first month of the first decade of the first century in the third millennium.