Chinese double hours and the split Rat hour

The twelve periods on a modern clock

Each branch corresponds to a two-hour interval. The following table presents the sequence within one midnight-to-midnight civil day, separating the two portions of Rat time. All intervals include their starting time and exclude their ending time. Thus 01:00 belongs to Chou, not early Rat, and 23:00 belongs to late Rat, not Hai.

Here 24:00 means 00:00 at the start of the following date.

The thirteen rows cover 24 hours. A continuous Rat period joins late Rat on one date to early Rat on the next. The two Rat rows within one civil date are separated by twenty-two hours and belong to different occurrences of Rat time.

Labels such as 00:00–00:59 and 23:00–23:59 express the same intervals to minute precision. They include the seconds within the final minute. There is no gap before the next period begins.

The split Rat hour and the day boundary

The clock interval and the day’s Ganzhi, or stem-and-branch name, are different questions. A time can belong to the Zi branch while a calculation either keeps the current day’s Ganzhi or advances it. The latter choice is especially important for the day pillar, the day’s stem-and-branch pair in a Bazi chart.

The Atlas’s engine distinguishes the following treatments of late Rat time.

These choices are not a single switch controlling every field. In particular, retaining the current day pillar does not make the engine retain that day’s stem for the hour calculation. The hour stem still follows the advancing convention.

Across midnight on September 11–12, 2026

September 11, 2026 is Wu-Zi (戊子) and September 12 is Ji-Chou (己丑). The following comparison uses a fixed UTC+8 clock.

The Jia-Zi hour continues across midnight. Early Rat at the beginning of September 11 was Ren-Zi, so the two Rat rows within that date share a branch but have different stems.

The hour stem follows a compact rule. Number the stems Jia = 0 through Gui = 9 and the branches Zi = 0 through Hai = 11. Take twice the remainder of the relevant day-stem index divided by five, add the hour-branch index, and keep the remainder after division by ten. At late Rat, the relevant stem is the following day’s. A Wu day gives Ren for early Rat; the change to Ji at 23:00 gives Jia for late Rat.

Daily Pengzu sayings and daily Yi and Ji, the activity recommendations, remain tied to their ordinary daily inputs. Hourly activities and the hourly presiding deity use the advancing day convention. A late-night entry can consequently combine the civil day’s daily information with an hour calculated using the following day’s stem and branch.

Clock time, place and historical records

The table maps branch hours onto a clock after a time standard has been chosen. It does not convert a timestamp between time zones. Nor does a branch name alone identify the standard used in a historical record. “Mao time” supplies an interval, whereas a modern timestamp may identify a particular minute with an explicit time-zone offset.

Local solar time and modern standard time also differ. Local solar time is tied to the Sun’s position at a particular longitude. A standard-time clock follows an agreed regional reference. The same event can therefore receive different clock readings under those systems.

An older almanac’s branch hours belong to the timekeeping practice of its place and period. The modern table is useful for understanding the twelve names, but an exact historical conversion needs the original date, location and time convention. It should not acquire a modern time-zone offset merely because the branch name is familiar.