Being helped by the research on the calendrical calculation system during the first fourteen years of king Sejong period (r.1419-1450), in around 1434 Joseon 朝鮮 astronomers could calculate four time-units, year, month, day, and hour. However, due to the obligation of ‘upholding the first month and first day’ (奉正朔) which is forced by the Tribute-investiture (朝貢冊封) relationship, in Joseon the vassal kingdom, they could not use the calendrical days (曆日, year, month, and day) according to the cal- culation results based on the location of Seoul. The calendrical days ought to be accord with those of the Ming China, the dominant kingdom, whose standard point located at Nanjing 南京. The local time which was measured in real time, however, was not confined by the obligation. In Joseon, they could use their local time which was measured on the meridian of Seoul. And they were able to calculate and use those local times, the time of sun- rise and sunset (日出入時刻) and the length of day and night (晝夜刻), on Seoul’s latitude, and which were distinct from those of Ming China. It was Joseon’s local time that Jagyeongnu (自擊漏, automatic striking clepsydra) automatically calibrated and struck.