I arrived at about 11:15 PM in Bangkok. Because of a snafu I didn’t get to my hotel till about 1 AM. The day I landed, the previous day, counted as day zero. I don’t know if there’s any official policy that says the day zero begins the day you land on the airport, but that was my experience
Such an important question, if I were in your shoes I would also contact the Thai Embassy. Some individuals in this group have valuable information, but how will you know?