Why are the due dates for my to-do's in iCal 1 day before the real due date ?

The Sandy iCal calendar I subscribe to shows incorrect information: a todo shows up one day before the event is due, e.g. a to-do planned for April, 28, will show up in iCal under Sunday 27 april.

My time zone is Europe/Brussels and has been configured correctly both in the Sandy settings, and in iCal.