If you include the current day then you can book for that day and the next 354 days (355 in total). Looks like it counts each day including the current day - today is day one and tomorrow is day two - rather than today being day zero and tomorrow being day one (24 hours later=1 day).
I suppose this this is why the phrase often used is that flights can be booked up to 355 days in advance (354 days, 23 hours, 59 minutes) up to the end of the full day.