docs: def duration(self):
          """Calculates the breeding cage's duration.
  
          This is relative to the current date (if alive) or the date of inactivation (if not).
          The duration is formatted in days."""
          if self.End:
              age =  self.End - self.Start
          else:    
              age =  datetime.date.today() - self.Start
          return age.days