1. The two hours from 23: on the same day to 1: on the next day are the sub-time, and the middle 24: (also called : ) is the boundary between the two days. Therefore, the sub-time is divided into the former sub-time and the latter sub-time, each of which is one hour, the former sub-time is on the previous day and the latter sub-time is on the next day, but the heavenly stems are the same. In ancient China, there was only the concept of "hour", but the concept of "hour" was introduced to China only in modern times by the west. Because there was no distinction between the preceding and the following, about half of the people born at the time of birth made mistakes in their birthdays, and when there was a mistake in their daily activities, the eight characters were bound to make subversive mistakes.
2. In a day, it can be divided into early hours (: to 1: ) and late hours (23: to 24: ). Although they are both sub-hours, their heavenly stems are different. If they are not distinguished, it will inevitably lead to mistakes.