Deriving dates

Hi all! I am looking at projects. I have different phases and duration for each of those phases, which vary per project. My first phase is definition. And my second phase (configuration) cannot start until definition has completed. I am currently using interval as the field for duration BUT ... I can't work out if there is a way to calculate the start date of my configuration based on the duration of my definition 

something like... Starts Date of Definition + interval duration = start date of configuration 

thanks a million! 

C