From Zurich to Sibenik there are buses driving year around, the buses departure from the bus and carparkplatz at sihlquai. the timetable for the buses you can see below, check the small info icon to get details about each departure. Travel time for the 1100 km long bus route from Zurich to Sibenik is about 17-18 hours
I you are going on holiday in Sibenik and has to return to Zurich, you should buy return ticket, you can check timetable for returning buses here