[HELP REQUIRED]Sort according to Expiry date

[lucky]

Hi,

I have few downloadable products in my prestashop.
Now I want to Sort them according to Expiry Date.

I guess the code in product-sort.tpl should be changed. But I am not sure what exactly should be changed.
Can someone help me?
I want to sort by Expiry date instead of price:

{l s='price: lowest first'}

-lucky