<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def save(self, *args, **kwargs):
 """ Fill 'created' and 'modified' attributes on first create """

        if self.created is None:
            self.created = tz_now()
        
        if self.modified is None:
            self.modified = self.created
            
        super(Thread, self).save(*args, **kwargs)