TL;DR: The Hour of the Rat is the block from 11 p.m. to 1 a.m. on the clock. The ancients divided a day and night into 12 watch-hours of 2 hours each, named after the 12 earthly branches from Tý to Hợi. There are two ways to set the day boundary at the Hour of the Rat: Bát Tự usually turns the day at 11 p.m., while some Tử Vi books keep the calendar date of birth, so someone born between 11 p.m. and midnight can get two different stem-and-branch days.
What time is the Hour of the Rat?
The Hour of the Rat (giờ Tý) is the block from 11 p.m. to 1 a.m. on the modern clock. It is the first of the 12 watch-hours, tied to the earthly branch Tý, whose animal is the rat; in the old view it is the stillest moment, when yang energy begins to arise.
There are two ways to set the day boundary at the Hour of the Rat. The first, commonly used in Bát Tự, turns the day at 11 p.m., the start of the Hour of the Rat: a birth at 11:30 p.m. takes the next day's stem and branch. Đẩu số tinh thành records a second way: within one calendar day, roughly 0:01 to 1:00 and 23:01 to 24:00 both count as that day's Hour of the Rat[1]. For a birth between 11 p.m. and midnight, the two ways can give different days; some Bát Tự practitioners also split the Hour of the Rat into an early and a late half.
Why does a day have only 12 watch-hours?
The ancients divided a day and night into 12 watch-hours of 2 clock hours each, named after the 12 earthly branches from Tý to Hợi. The division follows the twelve earthly branches of the old calendar and the natural rhythm of daily life: cockcrow, sunrise, noon, sunset, dusk, midnight.
In the image-and-number tradition (tượng số), the twelve branches also stand for the 12 months and 12 directions. Tý is due north, matching the 11th lunar month and midnight; Ngọ is due south, matching the 5th lunar month and noon. Hours, months and directions thus sit on one ring of branches.
Table of the 12 watch-hours in clock time
The table below matches the 12 watch-hours with clock time. This is a stable convention in East Asian calendar and thuật số books; the last column gives the folk image usually tied to each watch.
| Watch-hour | Earthly branch | Clock time | Folk image |
|---|---|---|---|
| Tý (Rat) | 子 | 23:00 to 01:00 | Midnight, all is still |
| Sửu (Ox) | 丑 | 01:00 to 03:00 | First cockcrow, still dark |
| Dần (Tiger) | 寅 | 03:00 to 05:00 | Tigers hunting, near dawn |
| Mão (Cat) | 卯 | 05:00 to 07:00 | Sunrise |
| Thìn (Dragon) | 辰 | 07:00 to 09:00 | Morning, breakfast time |
| Tỵ (Snake) | 巳 | 09:00 to 11:00 | Near noon, snakes leave their holes |
| Ngọ (Horse) | 午 | 11:00 to 13:00 | Noon, yang at its height |
| Mùi (Goat) | 未 | 13:00 to 15:00 | Early afternoon |
| Thân (Monkey) | 申 | 15:00 to 17:00 | Late afternoon, monkeys climbing |
| Dậu (Rooster) | 酉 | 17:00 to 19:00 | Dusk, chickens go to roost |
| Tuất (Dog) | 戌 | 19:00 to 21:00 | Evening, dogs guard the house |
| Hợi (Pig) | 亥 | 21:00 to 23:00 | Late night, pigs asleep |
A quick way to remember: every watch-hour starts at an odd hour, and the Hour of the Rat is the only one that spans midnight.
How does the Hour of the Rat in Tử Vi and Bát Tự differ from the ordinary one?
In Tử Vi and Bát Tự, the birth hour is not just a 2-hour block but a full pillar with both a heavenly stem and an earthly branch. The branch of the Rat hour is always Tý, while its stem changes with the stem of the day, worked out by the Five Rats rule (Ngũ thử độn, 五鼠遁).
Thần Phong thông khảo (神峰通考), a Tử Bình book by Zhang Nan (Trương Nam) of the Ming dynasty, records the Five Rats verse: "甲己还加甲，乙庚丙作初。丙辛从戊起，丁壬庚子居，戊癸何方发？壬子是真途"[2]. The five pairs are:
| Stem of the day | Stem and branch of the Rat hour |
|---|---|
| Giáp, Kỷ | Giáp Tý |
| Ất, Canh | Bính Tý |
| Bính, Tân | Mậu Tý |
| Đinh, Nhâm | Canh Tý |
| Mậu, Quý | Nhâm Tý |
From the Rat hour, the later hours of the day follow on through the stem cycle. For example, 1 March 2026 is a Giáp Tuất day: a birth at 0:30 that day carries the hour Giáp Tý; a birth at 11:30 p.m. on the same calendar day has, in Bát Tự, already passed into the Ất Hợi day and carries the hour Bính Tý.
This rule is a convention of stem-and-branch reckoning that Bát Tự uses to set the hour pillar; it is not an astronomical phenomenon.
Why is the Hour of the Rat treated as the start of a new day?
In many East Asian thuật số systems, the stem-and-branch day starts at the Hour of the Rat. The convention ties to yin-yang thought: the Hour of the Rat is when yang begins to arise, so it is treated as the start of a new cycle. In modern life the new day still starts at midnight by civil time.
Common questions when looking up lunar hours
Is the Hour of the Rat the same as midnight?
The Hour of the Rat contains midnight but is not only midnight. It runs from 11 p.m. to 1 a.m., covering both sides of midnight. Someone who says "born in the Hour of the Rat" without more detail is usually understood to mean born between 11 p.m. and 1 a.m.
Are the Hours of the Rat and the Horse symmetrical?
Yes. The Hour of the Rat (23:00 to 01:00) and the Hour of the Horse (11:00 to 13:00) are the two opposite ends of the 12-watch cycle. In yin-yang thought, Tý is where yin peaks and yang arises, Ngọ where yang peaks and yin arises; feng shui and Tử Vi often treat the Tý-Ngọ pair as the axis of the earth plate.
Why is each watch-hour 2 hours long?
Because the ancients divided a day and night into 12 equal parts. A day has 24 clock hours; divided into 12 watches, each watch is 2 hours.
Do lunar hours and clock hours always line up?
Not always. Watch-hours are a convention tied to the earthly branches, while clock time follows the civil time zone and can be adjusted. Lục Nhâm, volume 1: Nhập môn notes that when the government changes the clock, solar clock time changes with it, but the lunar hour never changes[3]. So anyone reading a birth time recorded in old papers needs to know which time zone it was written in. Vietnam now uses UTC+7, and the Hour of the Rat runs from 11 p.m. to 1 a.m. in that zone.
Nhật Nguyệt's suggestion for using the table
A suggestion from Nhật Nguyệt: when converting a birth time to a watch-hour, write down the exact clock time first, then look it up in the table. Anyone born between 11 p.m. and midnight should know that Bát Tự and Tử Vi may place them on two different stem-and-branch days.
