I logged this bug a couple weeks ago, but I'm not seeing it in the issue tracker for some reason...
Basically, if you change the due date of a task through WinMilk, it erroneously sets the due for 9 PM the previous day. So you'd set it for, say, 6 Sep 2011, and upon save it would immediately show up as 5 Sep 2011 both in RTM and WinMilk.
- Both RTM and WP7 are set to PST (GMT-08:00)
- Both RTM and WP7 are set to use 24-hour clock
- WP7 has Monday set as first day of week in the regional settings
Perhaps one of these two settings is having an effect on date calculation?