SSRS find max of converted date field

Hi

My SSAS source has a date field in this for mat dd/MM/yyyy which is stored as a string.

In ym report when I find the MAx of this field it is doing it by the day rather than the date.

I have tried the following but it doesn't seem to work it just errors on the report, how can I get this to work.

Thanks

=MIN(FORMAT(datevalue(Fields!DDMMYYYY.Value),"dd/MM/yyy"))

when converting say 28/10/2013 it is saying the date is not valid so I think it is trying to do mm/dd/yyyy