[Zope] how to calculate a duration ?

hello zopers, happy new year to all zopistas !! in my zclasses i have a property to store a birthdate (id 1969/06/17) I want to render the age of that person but if I do (ZopeTime()-birthdate).strftime("%Y") I get age in days (as described in zopebook), so i can't deduce the age ( problem w/ leap years....) I'd like to get a results in years/month/days any idea??? it can't be the first time this is asked thx in advance