Charlie, I'm on your side It should be quite easy (if a little lengthy) to explain if you relate the issue to counting in tens.

The first ten is: one two three four five six seven eight nine ten.

The next ten is: eleven twelve thirteen fourteen fifteen sixteen seventeen eighteen nineteen twenty.

Notice folks that we start counting the second group of ten with the number eleven.

Likewise if we count in hundreds it goes: 'one' 'two'...'ninety nine' 'one hundred' The next hundred starts at 'one hundred and one'

Counting in thousands is just the same. The first thousand (millennium) begins with 'one', the second thousand begins with, you've guessed it 'one thousand and one' and the next millennium therefore begins with 'two thousand and one'

Bert. themsthefactsandidontwanttohearanymoreargumentsaboutit