The time systems employed in ancient China and Japan are discussed. It is well known that both in ancient China and Japan 1 day was divided into 12 double hours, and the first double hour began at 23 hr local time. However, it is confirmed in this paper that in the Chinese Song dynasty the first double hour began at 0 hr local time. One day was also divided into 100 equal parts, called ke, and ke was subdivided by a time unit called fen. The number of fen in 1 ke varied from dynasty to dynasty. These numbers were clarified by analyzing the tables of daytime duration given in the official Chinese chronicles. In ancient Japan, the time units ke and fen were also used, but the lengths of both of them varied depending on the era. It has been found that all of the daytime and nighttime, the times of sunrise and sunset, and the lengths of shadows given in the official Chinese chronicles refer to a particular latitude of about 34.°5, and that the Japanese system adopted this Chinese tradition. Symmetry of the data in tables with respect to certain dates was also investigated in detail in order to examine how the dates of 24 qis were determined.