Determine Database Uptime
There are a lot of system views from which you can pull a lot of information. There's of course one that tells you the uptime of your database.
Recipe #1 - Query v_$instance to get the uptime
SELECT host_name, instance_name, TO_CHAR(startup_time, 'DD-MM-YYYY HH24:MI:SS') startup_time, FLOOR(sysdate-startup_time) days FROM sys.v_$instance;
This query will return some additional information like the host and database instance name as well as the number of days the database is running in the last column.
Recipe #2 - Query v$session to get database uptime
You'll often see articles telling you to query the session with SID 1, but this won't work everywhere. The following query seems to be more reliable, it checks the process with the name PMON, which is part of the database and reads its logon time:
SELECT TO_CHAR(logon_time, 'DD-MM-YYYY HH24:MI:SS') FROM v$session WHERE program LIKE '%PMON%'