Operation Date type in plsql developer

To judge the size of the time type or set the time in plsql, two commonly used functions are to_char(date,'yyyy-MM-dd'),to_date('2016-7-12','yyyy-MM-dd');

select t.*,t.rowid from S_ent_system t where to_char(validity_date_end,'yyyymmdd') > '2017%'

update S_ent_system set validity_date_end=to_date('2020-7-27','yyyy-MM-dd') where to_char (validity_date_end,'yyyymmdd') < '2017%';