why do you want to CONVERT it. why not just format it as you like
By the by in exel you should enter date as mm/dd/yy your date should have been entered as 8/28/09
Suppose 28/8/09 is already entered by somebody else as a text (not as a date) (assuming 28/8/2009 is in A1)
in B1 copy paste this formula
you will get a number. format this as date in any format.