Zi hour (子时, zǐ shí) starts at 11:00 p.m. because the standard 12-shichen system centers its first two-hour period on midnight. It therefore runs from 11:00 p.m. to 1:00 a.m. The period and the modern calendar date are two different layers: Zi can cross 12:00 a.m. even when a civil date changes there.
In brief: Zi is the first of the 12 Earthly Branch periods. Twelve two-hour periods cover 24 hours, and Zi is arranged symmetrically around midnight. That explains the familiar 11 p.m.–1 a.m. conversion; it does not, by itself, settle every traditional or astrological rule about when a new day begins.
Contents
- The standard Zi hour conversion
- Why the period begins before midnight
- Zi hour and the start of a new day
- How to use Zi hour with a birth time
- Frequently asked questions
The standard Zi hour conversion
The Hong Kong Observatory gives the following modern mapping for the first three shichen:
| Earthly Branch hour | Modern local time | Zodiac animal | Day position |
|---|---|---|---|
| Zi (子) | 11:00 p.m.–1:00 a.m. | Rat | Midnight-centered |
| Chou (丑) | 1:00–3:00 a.m. | Ox | Deep night |
| Yin (寅) | 3:00–5:00 a.m. | Tiger | Before dawn |
The remaining nine periods continue in two-hour steps until Hai (亥), 9:00–11:00 p.m. The complete table appears in our 12 Shichen guide and in the Observatory’s Heavenly Stems and Earthly Branches reference.
Calling Zi an “hour” can be misleading in English. In this context, it is a shichen: one of 12 periods in a day, each corresponding to approximately two modern hours.
Why the period begins before midnight
A 24-hour day divided into 12 equal periods produces 12 two-hour blocks. The standard branch-hour arrangement places midnight at the center of Zi and noon at the center of Wu (午):
- Zi: 11:00 p.m.–1:00 a.m., centered on 12:00 a.m.
- Wu: 11:00 a.m.–1:00 p.m., centered on 12:00 p.m.
This is easier to understand as a centered interval than as a modern appointment slot. “Midnight” names a point; Zi names the period around it. In the same way, Wu embraces the approach to noon and the hour after it.
The branch labels also connect midnight and noon with a wider calendrical vocabulary. The expression ziwu pairs the two opposite points and can appear in discussions of time, direction, astronomy, and traditional medicine. The underlying time structure should be understood before any later symbolic interpretation is added.
Historical readers should not assume that everyone used a wristwatch-like 11:00 boundary. Officials and specialists worked with instruments, placards, astronomical observations, water clocks, and local signals. The modern two-hour table is a clear conversion convention, not evidence of minute-perfect household time in every dynasty.
Zi hour and the start of a new day
Zi crosses the modern date boundary:
11:00 p.m. 12:00 a.m. 1:00 a.m.
|-----------------------|---------------------|
start of Zi civil date changes end of ZiThis creates a real interpretive problem. A birth record at 11:40 p.m. and one at 12:20 a.m. share the same branch hour, but modern civil calendars place them on different dates.
Different technical or astrological traditions may handle that crossing differently. Some calculations change the day pillar when Zi begins; others distinguish the portion before midnight from the portion after midnight; still others retain the civil-date boundary. These are calculation conventions, not something the two-hour lookup table can decide on its own.
That distinction matters:
- Timekeeping question: Which branch period contains 11:40 p.m.? Zi.
- Civil calendar question: Has the date changed at 11:40 p.m.? Normally no.
- BaZi calculation question: Which day stem should generate the hour stem? The answer depends on the disclosed method.
Our separate guide to early Zi and late Zi documents the disagreement without declaring one lineage universally correct.
How to use Zi hour with a birth time
For a general cultural lookup, follow four steps:
- Use the local civil time recorded at birth.
- If the time is from 11:00 p.m. up to 1:00 a.m., identify the branch as Zi.
- Record the civil date exactly as it appears on the source document.
- If you later use a specialist calculator, check its day-boundary and time-correction settings.
Do not silently convert every birth to Beijing time. A birth time is tied first to the place and clock convention in which it was recorded. Daylight-saving time, historical time-zone rules, uncertain records, and solar-time adjustments can matter near a boundary.
A trustworthy calculator should show the original input and state whether it uses:
- recorded civil time;
- a historical time-zone database;
- daylight-saving correction;
- longitude or apparent solar time;
- a 23:00 or 00:00 day-change rule.
If a source only says “around midnight,” the honest result is uncertainty, not extra precision.
Frequently asked questions
What animal is Zi hour?
Rat. Zi is the first Earthly Branch and corresponds to the Rat in the 12-animal cycle. See the complete Chinese zodiac hours chart.
Is 12:30 a.m. Zi hour?
Yes. In the standard conversion, Zi lasts from 11:00 p.m. to 1:00 a.m., so 12:30 a.m. falls within Zi.
Is 11:00 p.m. the start of the next Chinese day?
It is the start of Zi hour in the standard branch-hour table. Whether a specialist calendar or BaZi method also changes the day at that point is a separate convention and should be stated explicitly.
Why not make Zi run from midnight to 2 a.m.?
The widely published modern correspondence centers Zi on midnight, producing the 11 p.m.–1 a.m. interval. Historical systems and reckonings varied, so a source discussing a specific period should be read in its own context.
Does Zi hour prove anything about personality?
No. It identifies a traditional time period. Personality descriptions attached to Rat hour are symbolic interpretations, not scientific conclusions about a person born during those two hours.
A period around midnight, not a hidden clock rule
Zi starts at 11 p.m. because the shichen system describes the two-hour span around midnight. Once that centered structure is clear, the apparent contradiction disappears: the branch period can begin before midnight while the civil date still changes at midnight. For the full daily sequence, continue with The 12 Shichen.