Winter solstice
Helmer Aslaksen, a mathematician who teaches at the National University of Singapore and specializes in the Chinese calendar, explains that those who use 2698 BCE as a first year probably do so because they want to have "a year 0 as the starting point", or because "they assume that the Yellow Emperor started his year with the Winter solstice of 2698 BCE", hence the difference with the year 2697 BCE calculated by the Jesuits.