怎么求多个date型的平均时间

for example:13:21:10 SQL> select * from t1;A
-----------------
20040101 00:00:00
20040102 00:00:00
20040103 00:00:00已用时间:  00: 00: 00.16
13:21:19 SQL> select
13:21:21   2  (select avg(a-(select min(a) from t1)) from t1)
13:21:21   3  +(select min(a) from t1) 平均时间
13:21:21   4  from dual;平均时间
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20040102 00:00:00已用时间:  00: 00: 00.00