The day divides itself here: the wind comes up around noon and drops again in the late afternoon, so a morning and an afternoon are genuinely different sessions rather than an arbitrary split.
A window is four hours and costs about two thirds of a full day. It suits a two night stay, a spare afternoon before a boat, and anybody who only ever surfs the dawn.
A window that runs over half an hour into the next one is charged as a full day, because somebody else has booked that board.