Timestamps can be confusing and difficult to work with in oracle. It took me a little time to figure out but the following SQL will give you
SELECT (CAST(SYS_EXTRACT_UTC(systimestamp) AS DATE) - TO_DATE('19700101', 'YYYYMMDD')) * 86400 seconds , current_timestamp FROM dual
the UTC time in seconds from the epoch, as a floating point number.
60 * 60 * 24 for 60 seconds * 60 minutes * 24 hours = 86400