How to perform views date calculations using global math?
You can try Views PHP which allows you to add a php code field to a View. 

Otherwise, for a less "dangerous" solution, you can add a field to your content type for which you are creating the View, hide the field from the node creation form with css, hook_form_alter() or Display Suite, and create a Rule that calculates the new date when a user save new content. This field could be a Date field, or an Interval field if you want to store the interval between two dates.