code: def save_date(self):
          
          task_invitation = TaskInvitation.objects.get(self.task_invitation_key)
          wfi = task_invitation.instance
          if wfi.current_actor.exist and wfi.current_actor == self.current.role:
  
              dt_start = datetime.strptime(self.input['form']['start_date'], "%d.%m.%Y")
              dt_finish = datetime.strptime(self.input['form']['finish_date'], "%d.%m.%Y")
  
              task_invitation.start_date = dt_start
              task_invitation.finish_date = dt_finish
              task_invitation.save()
  
              wfi.start_date = dt_start
              wfi.finish_date = dt_finish
              wfi.save()
  
              title = _(u"Successful")
              msg = _(u"You've extended the workflow time.")
          else:
              title = _(u"Unsuccessful")
              msg = _(u"This workflow does not belong to you.")
  
          self.current.msg_box(title=title, msg=msg)
docs: Invitations with the same workflow status are deleted.
              Workflow instance and invitation roles change.