Use timestamp in query

Hello,
I have the following query in oracle (PL/SQL):

select username, target, v.start_time, v.last_update_time, round(100*sofar/totalwork,2) as percent_, opname,
     time_remaining from sys.v_$session_longops v where
       start_time >= to_date('15-Sep-2012') order by username, start_time

I want to filter records than start_time > 15-Sep-2012 10:30 AM   but I don't know how to include time in above query.

Thanks for your help,
-FA