code: public Subscription endTrialAt( Subscription subscription, Date date ) {
      ParameterMap<String, String> params = new ParameterMap<String, String>();
      params.add( "trial_end", String.valueOf( date.getTime() / 1000 ) );
      return RestfulUtils.update( SubscriptionService.PATH, subscription, params, false, Subscription.class, super.httpClient );
    }
docs: Stop the trial period of a subscription on a specific date.
  
  @param subscription the subscription.
  @param date the date, on which the subscription should end.
  @return the updated subscription.