Gerd,
It sounds like you are addressing this to Rover. Unfortunately, they do not moderate this board, comprised of sitters and other owners.
However, your question is easily answered. Nothing you input is final; you will not be charged or held to any dates. Just enter your best guess and ask the sitter for a Meet & Greet. As a sitter myself, I always ask prospective clients to come over with their pet so that we can discuss the engagement, our pets can meet, and clients can see where their pet will stay.
Nothing is finalized. If you decide to go ahead with a booking, after meeting with the sitter, then the sitter formally "accepts" the booking, which lands it back in your court to finalize payment. At any stage, the booking can be modified to formalize the actual dates you need. So you should have no fear of being locked into anything by merely contacting a sitter.