I'm having a similar problem but none of my tasks are shared.

Example: I have a repeating task of "pay rent" set to repeat every month on the 25th, but due date is listed as the 24th. A month ago it seemed fine.

This does not seem seems to affect every task. The ones I notice are repeating ones.

Looks like I was able to resolve this myself. I had recently installed Windows 7 and set the time 12 hours ahead (10pm rather than 10am) so the program got confused about when the day ends. Give that a try, ken.goodridge and if you haven't already, look over the other "self help" solutions:

http://www.rememberthemilk.com/help/contact/support/?ctx=tasks.wrongday