Return a string representing the age of the process.

def birthday(self):
        """
        Return a string representing the age of the process.
        """
        if isfile(self.pid_file):
            
            # Timestamp / timezone string
            tstamp  = datetime.fromtimestamp(self.created())
            tzone   = tzname[0] 
            weekday = WEEKDAY[tstamp.isoweekday()]
            
            # PID file age / age string
            age     = self.age()
            age_str = '{}s ago'.format(age[0]) 
            
            # Birthday string
            bday   = '{} {} {}; {}'.format(weekday, tstamp, tzone, age_str)
            
            # Return timestamp / timestamp string
            return (self.created(), bday)
        return (None, None)