Oracle Database Discussions

I get the error message: ORA-01843: not a valid month after executing a SQL plus script. I try using the "standard" date format yyyy-mm-dd. Is SQL/PL not understanding the altered session statement?

set linesize 200
set pagesize 1000
alter session set NLS_NUMERIC_CHARACTERS = ',.';
alter session set NLS_DATE_FORMAT = 'yyyy-mm-dd';

select
 *
from my_table
where  
date >= '2019-08-31';

exit