"Zi" period: 11:00-01:00. Because during the "zi" period, it's deep at night and rats come out to move around, so it's called "zi shu" (rat of the zi period). Although the "zi" period starts at 11 PM, ancient people also counted 0 AM as the beginning of the next day, which was called "zi shi ban". Therefore, although the ancient timekeeping system differs from the modern 24-hour system, they do not contradict each other, as both consider 0 AM as the start of a new day.

The "zi" period refers to 11 PM to 12 AM. The period starting at 11 PM of the same day is called "night zi", and the period after 12 AM is called "early zi". The early zi and night zi should be considered as two different days.
Early zi: between 11 PM and 12 AM; Night zi: between 12 AM and 1 AM.

Some I Ching fortune-tellers say that people born in the "early zi" period have their "day" calculated as the same day, while those born in the "late zi" period belong to the next day. In fact, this is an idea created by later I Ching fortune-tellers who forgot that "the theory of fate was created in ancient times, but there was no 24-hour system back then—only the 12-period system." In other words, this is just a self-explanatory statement resulting from a lack of understanding of the difference between the "modern time system" and the "ancient time system."