User Tenure or Date Diff : Creating Cohorts or User Properties

I'd like to be able to classify users into cohorts (or apply a custom user profile property). 
I'd like these cohorts (or properties) to hold the calculating of TTC at the user level, in days. 


Let's say user does Event A on Jan 1, and Event B on Jan 20… I'd like to make a custom property or cohort that is essentially:

DateDiff ( Date of Event B - Date of Event A) = Age

or

Jan 20 - Jan 1 = 19 days old