As we know that is is impossible to calculate average on non numeric column.
SQL> select avg(hire_date) from scott.emp;
It will raise error
ORA-00932: inconsistent datatypes: expected NUMBER got DATE
Workaround is to convert date to julian date and then calculate average.
So the following is solution to make average on date column
SELECT TO_DATE(TRUNC(AVG( TO_CHAR(hire_date,'J'))),'J') FROM EMP;