One Piece creator Eiichiro Oda, in the latest SBS Q&A session, included in volume 113 of the manga, confirmed that two calendars exist in the series’ world: the Tenreki (Celestial Calendar) and the Kaienreki (Sea Circle Calendar). Each tracks time differently. The former is used by Sky Island residents and the latter by those living across the seas. In the same Q&A, Oda also confirms some semi-serious allegations that have been coming from the fanbase for a while now.
