Date format change

Dear Friends,
Need your help once again,

I have a variable ( e.g. ${i}) whoch has date in MM/DD/YYYY (E.g. 12/31/2011) format.
I want to change it to DD/MM/YYYY (e.g. 31/12/2011) format.
Request you to guide me as we are unable to do the same.
Thanks in advance
Anu.

$ echo "12/31/2011" | awk -F/ '{print $2"/"$1"/"$3}'
31/12/2011