See Two-Thousands AKA the 2XXXs
A thousand year period.
Not to be confused with the Third Millenium, which begins and ends exactly a year later.
The 2000s begin on January 1st of the year 2000 and end on December 31st of the year 2999 @ 11:59 P.M.
Source: Miskatonic Jack 2, The Ohio River Valley