Query Formula using last Relate record to Check and Uncheck a box

In my Project Manager APP,  I have a User Field Associated with Projects.  I want to calculate the time In between when every User Previous Project ends to When their New Project Starts. 

So the New Project Start Date - the previous Project end date when assign to the same Person.  Record 2 (assigned to this person) Project Start Date - Record 1 (assigned to this same person) Project End Date.