Just stumbled on to this in GW 8. We're running the Netware Agents (NLM).
Using the GroupWise 8 client in Windows XP, Program Release 8.0.1 1/8/2010.
In the GroupWise calendar, I put it in the Month View mode.
I select a day of the month by double clicking on it to bring up the Day Schedule view. At this point if I select either arrow next to the date at the top to move forwards or backwards one day, the date changes but NOT the day of the week.

For example if I open the Day Schedule for April 5, 2010, a Monday, and then click on the right arrow to move to Tuesday April 6 2010, the date changes to 4/6/2010, but the day stays as Monday. If I move forward another day, the date changes to 4/7/2010 and the day changes to Tuesday, still a day behind.

If I post an appointment after moving a day forward the appointment is put in the calendar based on the day not the date.
I get similar nonsense if I go backwards.

I only get this to happen if I have the calendar in Month view. Anyone else seeing this? I can reproduce it on multiple machines here.