The day trip works in both directions
Leave Rønne at 06:30 and you are in Ystad at 07:50 with a return as late as 22:30 – a good fourteen hours in Skåne. The other way round, the 08:30 from Ystad puts you in Rønne at 09:50 with returns until 20:30. In both cases the bottleneck is the last evening sailing, not the first one in the morning.
FlashFerry