Security :: Logon / Off Audit Using Audit_Trail?

getting logon timestamp for our auditing process. In some website, it says to get the logon timestamp I have to select it from timestamp column of dba_audit_session but when I do this, some results has a logoff earlier than logon. Is timestamp column really the logon timestamp?

SELECT
returncode,
(logoff_time-timestamp) timeonline,
TO_CHAR(timestamp, 'DD-MON-YYYY HH24:MI:SS') LOGON_TIME,
TO_CHAR(logoff_time, 'DD-MON-YYYY HH24:MI:SS') LOGOFF_TIME,
action_name
FROM dba_audit_session;

[code]....