The title of a book written by Gim Il-bu in 1884–1885. Jeong means “right” or “correct” and yeok means “change,” and hence the book title translates as: “Right Change.” In Jeongyeok, Gim Il-bu systematized the principles underlying the Later Heaven Gaebyeok and revealed that the calendar year would transform to 360 days per year in the Later Heaven.