Timezone for DUE in allday events is unhandled in calendar.xml script (
src/syncevo/configs/scripting/11calendar.xml )

> if (ITEMDATATYPE()=="vCalendar10" && ALLDAYCOUNT(DUE,DUE,TRUE,TRUE)>0) {
>     DUE = DATEONLY(DUE);
> }

should probably have:
>     DUE = CONVERTTOUSERZONE(DUE);
before DUE = DATEONLY(DUE);

Current code leads to wrong due date of a todo task (at least for me and GMT+2
on both the phone and the pc):
> [2012-09-23 12:21:22.795] - Field: DUE = 2012-10-02T22:00:00Z (TZ: UTC) (timestamp)
> [2012-09-23 12:21:22.795] - Expression result: 2012-10-02T22:00:00Z (TZ: UTC) (timestamp)
> [2012-09-23 12:21:22.795] - Parameter #1 (by value) = 2012-10-02T22:00:00Z (TZ: UTC) (timestamp)
> [2012-09-23 12:21:22.795] - DATEONLY() function result = 2012-10-02 (floating) (timestamp)
> [2012-09-23 12:21:22.795] - Assigned expression result = 2012-10-02 (floating) (timestamp)

BTW - thank you for your effort on the project.