Zi Hour and Time Boundaries: Dividing the 12 Double-Hours

子时 is the first of the twelve double-hour divisions of the day, and its position at the junction of the calendar day makes it one of the most consequential boundaries in Chinese-metaphysics practice. The hour itself is fixed by clock time, but its date attribution has been understood in more than one way. The entry below sets out the standard clock correspondence, the reason 子时 crosses midnight, the disagreement over whether the day changes at 23:00, how to treat a birth falling on the boundary, and which pillars are affected by the difference.

十二时辰与钟点的对应

One 时辰 equals two hours, and twelve 时辰 make one full day and night. The sequence begins with 子时 at 23:00 and runs to 亥时 at 21:00–23:00.

The standard correspondence is:

• 子时：23:00–01:00
• 丑时：01:00–03:00
• 寅时：03:00–05:00
• 卯时：05:00–07:00
• 辰时：07:00–09:00
• 巳时：09:00–11:00
• 午时：11:00–13:00
• 未时：13:00–15:00
• 申时：15:00–17:00
• 酉时：17:00–19:00
• 戌时：19:00–21:00
• 亥时：21:00–23:00

Thus each 时辰 is a fixed two-hour band. The first half of 子时 falls before midnight and the second half after it, but both halves belong to the same 时辰.

子时为何跨过午夜

子时 is the only one of the twelve 时辰 that crosses midnight. From 23:00 to 23:59, the time still belongs to the preceding Gregorian calendar day; from 00:00 to 01:00, it belongs to the following calendar day. The 时辰 itself, however, is continuous. This means that 子时 contains within it the formal change of the civil date.

Because the civil date changes at 00:00, while 子时 begins at 23:00, the same 时辰 straddles two dates. That structural fact is the source of the boundary problem: if one treats the date as changing only at midnight, then the first hour of 子时 belongs to the old day; if one treats the date as changing at 23:00, then the whole of 子时 belongs to the new day.

二十三时换日的两种说法

The question of whether the day changes at 23:00 has been answered in two ways in older sources. One position holds that the day changes at 23:00, so a person born at 23:00 or later is assigned to the next day. The other position holds that the day changes at midnight, so the period from 23:00 to 23:59 remains part of the current day.

This is not a minor matter of labelling. Under the 23:00-change convention, a birth at 23:30 is treated as occurring on the next day for the purpose of the 日柱. Under the midnight-change convention, it is treated as occurring on the current day. Both readings appear in traditional material, and the difference persists in contemporary tools.

The site’s position is as follows: for a birth from 23:00 to 23:59, the 日柱 is advanced to the next day, while the 时柱 is still calculated from the actual clock time within the 子时 range. The date change does not alter the 时支; a birth at 23:30 still has its 时支 in 子, because 子时 runs from 23:00 to 01:00.

落在时辰边界上的处理

When a birth falls exactly on a 时辰 boundary, the first step is to determine which two-hour band contains the clock time. A birth at 01:00 belongs to 丑时, not to the final moment of 子时, because 丑时 begins at 01:00. The same principle applies at 03:00, 05:00, and every other boundary. The clock reading determines the 时支.

For the 日柱, the boundary under the site’s convention is 23:00, not 00:00. A birth at 23:00 or later is treated as belonging to the next day for the purpose of the 日柱, even though the civil date has not yet changed. A birth at 22:59 remains on the current day; a birth at 23:00 is on the next day.

Because 子时 runs from 23:00 to 01:00, a birth at 23:30 and a birth at 00:30 share the same 时支. They differ in 日柱 under the midnight-change convention, but under this site’s convention they are treated alike: both have a 时支 of 子 and both are assigned to the next day’s 日柱.

这一处分歧影响哪几柱

The 23:00-change question directly affects the 日柱 and the 时柱. It does not affect the 年柱 or the 月柱, which are governed by different boundaries.

The 年柱 changes at the 立春 solar term, not at 23:00 and not at the civil New Year. A birth at 23:30 after 立春 has already changed year-pillar, but that change is caused by 立春, not by the hour. A birth at 23:30 before 立春 still belongs to the previous 年柱. The 月柱 changes only on the twelve major solar terms, the 节, not on the first day of the Gregorian month and not at 23:00. Thus 立春 changes the 年柱 only; 23:00 changes the 日柱 and the 时柱.

The 时柱’s 地支 is fixed by the clock time. Its 天干 cannot be determined independently. The 时干 must be derived from the 日柱’s 天干 by the 五鼠遁 method. This means that any change to the 日柱 can change the 时干, even when the 时支 stays the same.

五鼠遁 sets out five correspondences between the day-stem and the first 子-hour stem:

• 甲 day and 己 day begin with 甲子
• 乙 day and 庚 day begin with 丙子
• 丙 day and 辛 day begin with 戊子
• 丁 day and 壬 day begin with 庚子
• 戊 day and 癸 day begin with 壬子

Once the 子时 stem is fixed, the remaining 时辰 follow the 六十甲子 order. Each step forward by one 时辰 advances both the 天干 and the 地支 by one position. On a 甲 day or a 己 day, 子时 is 甲子，丑时 is 乙丑，寅时 is 丙寅, and so on through to 亥时 as 乙亥.

The input to 五鼠遁 is the 日柱’s 天干, not the calendar date. For a birth after 23:00, the 日柱 has already advanced to the next day under this site’s convention. If one used the previous day’s 日干 to apply the formula, the entire 时柱 would shift by one position. The error would not be confined to the 时干; the resulting 时柱 would be wrong as a whole.

A practical way to detect which convention a tool uses is to test a birth at 23:30. Under the 23:00-change convention, the 日柱 and the 时柱 are both calculated from the next day’s 日干. Under the midnight-change convention, the 日柱 remains on the current day, and the 时干 is calculated from the current day’s 日干. The two outputs differ, and the difference shows which of the two readings the tool has implemented.

The four pillars therefore do not share a single boundary. The 年柱 uses 立春, the 月柱 uses the twelve 节, the 日柱 uses 23:00, and the 时柱 uses the fixed two-hour bands. These boundaries operate independently, and a birth falling near 子时 may cross more than one of them at once.