update hour in date field

Hello,
I need to update the hour part of a date field, say the field name is createddate.

Like if its value is 17-SEP-2007 21:00:00, I need to update it as 17- SEP-2007 20:00:00 and if its value is 17-SEP-2007 22:40:00, then it be updated as 21:40:00

In other terms need to reduce the hour field value by 1 for all rows in the table.

Please advise. I am using Oracle 9i. Thank you very much. Received on Mon Sep 17 2007 - 00:02:14 CDT