Robin - Since when has "twenty" been a contraction of "two-thousand"? . For about as long as "nineteen" has been a contraction of "one thousand nine hundred".

Autolycus - as for "the century" starting in 2001 not 2000. Who says they all have to be the same length? If you can accept a year as being 365 or 366 days, and a month as having anything from 28 to 31 days, why could not the 1st century be deemed to have had only 99 years (reverse of a leap-century), so that every subsequent one started on 01-01-00.