In 10g there is a view called DBA_HIST_DATABASE_INSTANCE which has database instance information. It has history on information such as instance startup time, last ASH sample id.

Example:
SQL> column startup_time format a35
SQL> alter session set nls_timestamp_format=’DD-MON-YYYY HH24:MI:SS’;

Session altered.

SQL> SELECT startup_time, last_ash_sample_id FROM dba_hist_database_instance WHERE dbid = 1992878807 order by startup_time;

STARTUP_TIME LAST_ASH_SAMPLE_ID
—————————— ——————
12-AUG-2009 21:12:19 447735
21-AUG-2009 17:48:29 494755
..
27-SEP-2009 15:24:04 3036072
29-SEP-2009 21:44:22 0

8 rows selected.