Date Jump in SLA

We have a normal priority with an SLA completion time for 8 hours and it has been working fine until today, now when you select normal it sets it to 16 hour completion time

Example

Request ID : 681 Date Created : 29-Nov-2005 13:33 DueBy Time : 01-Dec-2005 13:03

It should be due by 30-Nov-2005 13:33

we are using

Version : 4.1.0
Build Number : 4112

Any help would be great

Thanks
Scott