How Chinese Calendars Describe the Qualities of Days

Two interlocking cycles

The stems and branches are two interlocking cycles. The ten stems and the twelve branches are paired off in a fixed sequence, and because ten and twelve run out of step, a given pairing does not recur until sixty steps have passed. This is the sexagenary cycle, the count of sixty, which the Chinese have used since ancient times to number days, and later months, years, and the two-hour periods of the day. Every day, then, carries a two-part name, one stem and one branch, and this name is the first thing a reader of the tradition notices about it. Because the cycle is continuous and unbroken, running on regardless of the calendar's other divisions, it provides a steady framework of recurring qualities.

Counting the sixty-day cycle

One aspect of this machinery deserves separate notice, because it is the part of date selection that is objectively verifiable and genuinely impressive.

The sixty-day cycle runs continuously. It is not reset at the new year, not adjusted for leap months, and not tied to the lunar or solar calendar at all: day sixty is followed by day one, without interruption, indefinitely. This makes the day count independent of every calendar reform, and Chinese calendars have been reformed many times.

The remarkable claim about it is that the sequence has run unbroken for a very long time. Day designations recorded in the Spring and Autumn Annals can be cross-checked against the solar eclipses that text also records, and the eclipses can be dated independently by modern astronomy. The dates line up. The stem-and-branch day count is therefore one of the longest continuously maintained chronological records anywhere in the world, running from at least the eighth century BCE to the present without a demonstrable break.

It is worth being clear about what this does and does not establish. It establishes that Chinese record-keeping was extraordinarily disciplined, and it makes the day cycle a genuinely useful tool for historians dating documents. It establishes nothing whatever about whether the days so numbered differ in quality. An unbroken count is an achievement of bookkeeping, not evidence about the days being counted.

Where a day’s qualities come from

Those qualities come from the associations the tradition attaches to the stems and branches. Each stem and each branch is linked with one of the five phases, wood, fire, earth, metal, and water, and with yin or yang. A day therefore carries a small cluster of elemental associations: the phase of its stem, the phase of its branch, and the balance of yin and yang between them. The branches carry further associations as well, including the familiar twelve animals and a set of directional and seasonal meanings. From these, the tradition reads a day's basic disposition, whether its energies are rising or settling, whether it leans towards one phase or another, and how it stands in relation to other days.

Judgement is always relational

The heart of the method is relationship. A day is rarely judged in isolation; it is judged in relation to something else, most often the person or the purpose for which the day is being chosen. The branches in particular are held to stand in fixed relationships with one another: certain pairs are said to combine harmoniously, while others are said to clash or to harm. A day whose branch clashes with the branch of a person's birth year, for example, is treated with caution for that person, however favourable it might be in general. In this way the same day can be suitable for one family and unsuitable for another, and the abstract cycle of stems and branches becomes a tool for matching a moment to an individual.

The clash, and the person it excludes

Of all the relational judgements in date selection, one is met by ordinary people more often than any other: the chong (冲), or clash.

The twelve branches are arranged so that each stands directly opposite another, six positions away on the cycle — rat opposite horse, ox opposite goat, and so on through the six pairs. A day's branch is therefore always in opposition to exactly one of the twelve zodiac animals, and the almanac prints this on every entry.

The practical rule is that the person born under the clashed animal should not take the leading part in whatever is being done that day. At a wedding this excludes them from prominent roles; at a funeral, from certain duties; at a groundbreaking, from turning the first earth. The rule is about a role rather than about attendance, though families vary in how strictly they read it.

This is worth understanding for two reasons. First, it is the point at which date selection stops being about days and starts being about particular people, which is why a date chosen for a wedding may have to be checked against the birth years of both families. Second, it is where the practice most often causes avoidable hurt. A relative told they cannot stand at a wedding because of the year they were born has been excluded from a family occasion on grounds nobody can examine, and the distress this causes is entirely real even though the clash is not.

Solar terms and the month's own character

A day is never judged in isolation from the season, and this is where date selection makes contact with the solar year rather than the lunar one.

The year is divided into twenty-four solar terms (节气), fixed by the sun's position along the ecliptic and therefore falling on nearly the same Gregorian dates each year. Twelve of these mark the beginnings of the months used in this system, so that the "month" for date-selection purposes begins not at the new moon but at a solar term — Beginning of Spring, Waking of Insects, Clear and Bright, and so on. Each such month carries its own branch, called the yuejian (月建), and the day's character is read against it.

The consequence is that the same stem-and-branch day is not equivalent to itself at different times of year. A day whose branch matches the month's is strengthened; one that clashes with it is weakened. This is the same logic that governs BaZi, where the month branch is the single most important element in a chart, and it is one of the clearest illustrations of how thoroughly these arts share a common machinery.

It also explains a source of confusion for new readers. Almanac reckoning uses two different calendars at once: lunar dates for festivals and for naming the day of the month, solar terms for determining which month a day belongs to. When the two disagree near a boundary — and they frequently do — different editions may assign the same day to different months, with different judgements following.

Longer patterns

The stems and branches also generate the rhythm of longer patterns that date selection draws upon. Because each month and each year also carries its own stem-and-branch name, a day can be read against the month and the year in which it falls, and the interplay of these layers, day within month within year, adds further texture to the judgement. A day that would be ordinary in one month may be strengthened or weakened by the month's own character; the tradition attends to these combinations, seeking days whose several layers agree rather than conflict. Much of the apparent complexity of the specialist's art comes from tracking these overlapping cycles at once.

These longer patterns raise a question the tradition does not settle: how many layers should be counted before a judgement is made? An almanac entry may be the product of half a dozen cycles running at different speeds, and there is no rule fixing which takes precedence when they disagree. In practice the compiler decides, and different compilers decide differently. The system supplies many considerations and no method for weighing them, which is the same structural feature that appears in every art in this section.

The single underlying idea

For the reader it helps to see that this whole apparatus rests on a single underlying idea: that time has a structure, expressed in recurring cycles, and that the qualities of a moment can be read from its place in those cycles. It is an elegant idea, and within the tradition it is applied with real rigour and consistency. Given the same rules and the same day, two practitioners will describe the day in the same terms. That internal consistency is one reason the system has commanded respect for so long.

Consistency is not truth

It is important, though, to distinguish the consistency of the method from the truth of its claims. That the stems and branches recur in a regular sixty-day cycle is a plain fact of the counting system. That the day named by a particular stem and branch actually carries the elemental qualities and relationships the tradition assigns to it, and that these could influence the success of a marriage or a journey, is a belief of the tradition, not a demonstrated fact. The cycles are real as a way of numbering time; the qualities read into them belong to a classical cosmology rather than to anything modern knowledge can confirm. Understood in that light, the stems and branches are best appreciated as an ingenious and coherent framework of meaning, the grammar in which date selection is written, rather than as a measurement of hidden forces in the days themselves.

It is worth separating two things that are easily run together here. The calculation is checkable: anyone with the rules can work out which stem and branch a given day carries, and everyone who does it correctly will get the same answer. The interpretation is not: no procedure exists for confirming that a wood day favours beginnings, and the tradition offers none. Almanacs present both on the same line, in the same typeface, with the same air of authority, and the reader is given no signal that they are different kinds of statement.

Holding that distinction is most of what is needed to read this material well. The cycles are a real and elegant piece of calendrical engineering, worth understanding on their own account. The qualities assigned to their positions are a system of convention, interesting as cultural history and unsupported as description of the world.

Key points

• Beneath the almanac’s ready-made judgements lies the stem-and-branch reckoning of time.

• Ten stems and twelve branches interlock, so a pairing recurs only every sixty units.

• Each stem and branch carries phase, polarity and seasonal associations that give a day its character.

• A day is judged in relation to something — a person, a purpose, a direction — rather than in isolation.

• The same cycles generate the longer monthly and yearly patterns date selection uses.

• The underlying idea is that time has structure, and that some moments suit some actions better than others.

• The method is internally consistent, but consistency is not evidence that the judgements are correct.

Sources drawn on for this article include: Joseph Needham, Science and Civilisation in China, on the sexagenary cycle and the Chinese reckoning of time; Richard J. Smith, Fortune-tellers and Philosophers: Divination in Traditional Chinese Society; and general studies of yin-yang and the five phases as they apply to the stems and branches.