I tried reverting to 23:50 June 30th and it did not hanged when switching to July 1st. I've tried it a few times without any problems. So I deactivated all NTP related jobs, switched the date back to June 30th 23:10 and reboot the system. Wait and see.
I am also using Debian... But Sarge 3.1. There might be a relation there unless somebody comes up having the same problem using another dist. At least it's not CFS related.
All my server/workstations are in sync using ntp... And yes, the year is set properly on all of them.