SQL> col today new_value today

SQL> select to_char(sysdate,'YYYY-MM-DD') as today from dual;

TODAY
----------
2018-04-23

1 row selected.

SQL> def today
DEFINE TODAY           = "2018-04-23" (CHAR)

SQL> accept dt date format YYYY-MM-DD default &today prompt "Enter a date (press ENTER for today's date): "
Enter a date (press ENTER for today's date):
SQL> def dt
DEFINE DT              = "2018-04-23" (CHAR)