col start_time new_value start_time
col end_time new_value end_time

select to_char(sysdate,'mm/dd/yyyy hh24:mi:ss') start_time
from dual;

{stuff}

select to_char(sysdate,'mm/dd/yyyy hh24:mi:ss') end_time
from dual;

select (to_date('&end_time','mm/dd/yyyy hh24:mi:ss') -
        to_date('&start_time','mm/dd/yyyy hh24:mi:ss')) *24*60*60 total_time_in_seconds
from dual;