Early Zi hour refers to the period between 11 PM and midnight;
Late Zi hour refers to the period between midnight and 1 AM.
According to ancient people, "Zi" means "growth," referring to the萌芽 of all things under the initial movement of yang energy. This perfectly matches the state of the Zi hour, where the first yang energy begins to arise.
Therefore, the ancients regarded the Zi hour as the starting point of the day, meaning that the new day begins from the Zi hour. According to modern timekeeping methods, the Zi hour is from 11 PM to 1 AM the next day. Modern people usually believe that a new day begins after 12 AM tonight. However, in the view of the ancients, a new day has already begun after 11 PM tonight.

Firstly, the two hours from 11 PM to 1 AM on the same day are considered the Zi hour, with 12 AM (also called 0 AM) being the boundary between two days. Therefore, the Zi hour is divided into the earlier Zi hour and the later Zi hour, each lasting one hour. The earlier Zi hour belongs to the previous day, while the later Zi hour belongs to the next day, but both have the same heavenly stem. Ancient China only had the concept of "hour" (shi), while the concept of "hour" (xiao shi) was introduced from the West in modern times. Because there was no distinction between the earlier and later Zi hours, about half of the people born in the Zi hour had incorrect birth dates, and once the day stem and branch were wrong, the eight characters would inevitably be completely off.
Secondly, when considering the day, it is also divided into the early Zi hour (midnight to 1 AM) and the late Zi hour (11 PM to 12 AM). Although they are both Zi hours, their heavenly stems are different. If not distinguished, and the early Zi hour is used universally to determine the hour stem, errors will inevitably occur.