If that was the date were currently, the answer would be "today".
AUXXV
Today
It is: TODAY
Today
Today
eighteen August two thousand eleven
today
today
Providing that is the current date; Today
Xix viii mmxi (you cannot write it in 9 letters only possible with 11 letters)
You cannot do it (Answer dated: 27 Aug 2011).
As long as you read this on 16 August 2011: "Today" If you read this on 17 August 2011, you'll need nine letters: "Yesterday" If you read this before 23 August 2011, you'll need 11 letters and 1 space: "Last Tuesday" Beyond that I give up.