text: -- date time functions
  
  -- [SPARK-16836] current_date and current_timestamp literals
  select current_date = current_date(), current_timestamp = current_timestamp();
  
  select to_date(null), to_date('2016-12-31'), to_date('2016-12-31', 'yyyy-MM-dd');
  
  select to_timestamp(null), to_timestamp('2016-12-31 00:12:00'), to_timestamp('2016-12-31', 'yyyy-MM-dd');
  