From Sibenik to Zadar there are plenty of bus connections year around, the travel time by bus is about 1 to 1.5 hours, majority of buses follow the coastal road. The bus departure from the main bus station in Sibenik
If you’re planning to do a daytrip to Zadar and return back to Sibenik, you can get a discount if you buy a return ticket where you travel both ways with the SAME Company, this would give you less departure to choose between. You can see return timetable from Zadar to Sibenik here.