What exact expression are you using to auto-enter a date?
I am just using a date entered into the data box:
So when I create a new record the date "27/08/2010" is shown in the field but it is not recognised as a date (my age calculation doesn't recognise this date) unless I manually type it in.
In a similar field where I enter a date into a data box, "27/08/2010" comes up as "8/27/2010".
I suspect that is being treated as text instead of a date.
Try the calculation option and use: Date ( MM ; DD ; YYYY ) and see if it works for you.