Calculate Date based on form popup entry?

[kensonplays]

For a forum I am a single-board mod on, I'd like to configure something where we put in the date the package was approved, then add X days and output that as the reply. I am unsure on how to do this. I could figure out how to output X days, of course, but I'm unsure how to calculate, for example:

"package approved on April 20th" with the next date they can submit a new package request as 15 days after April 20th.

Since the completion post isn't posted until after everything's done, and the new date is based on when it was approved.