1. Create an Action macro to set the Call Back Date to 13 days from now.
I have attached an example you can pdm_userload (make sure that Spell Macro table row with id 40001 / persid "macro:40001" does not already exist).
2. Create an Event that will execute the macro.
3. Attach the Event to the ticket by some means (Auto Event, Service Type, etc.) when the ticket is created.