Custom replacement pattern for expiration date

We are using the Drupal Conference Module. We would like the add expiration date to the email that gets automatically sent to reviewers. See Screenshot. As you can see the only 'variables' available to me are: %title, %conference, %username, %download.

How can I create a custom variable with a review expiration date. Eg. %reviewExpiration= assignedDate + 15days. The 'assignedDate' can be the date in which this notification was sent( or current date?).

Any ideas?

Thanks in advance.

Ce