This trip was a major concern because the confirmation and voucher clearly stated the tour commenced at 4.00pm. When we rang to confirm, as advised on the confirmation voucher, they agent conducting the tour said the tour actually started at 9.30am, which was too early for us to make it to the meeting place.
Eventually after much negotiation we arranged to have the guide meet us at 4.00pm as there was another couple whose documentation from Viator had the wrong time written on it. The tour operator did say that this was not an unusual situation with Viator. May I suggest you do something about getting the timing right on your documents.