SQL & PL/SQL

Good day,

I'm looking for a way to round the time portion of a date value. My query needs to return the time of day, rounded to the nearest 30 minutes - top of the hour or bottom of the hour.

If sysdate is: 12/28/2009 11:08:19 AM
I need to see: 12/28/2009 11:00:00 AM

If sysdate returns: 12/28/2009 11:18:19 AM
I need to see: 12/28/2009 11:30:00 AM

Any help is appreciated.

Regards,

Gary F.