What is the lunisolar calendar

From tortoise shells to the year-count

Oracle-bone inscriptions at Yinxu show that the Shang already recorded full and deficient months (30 / 29 days) and ordinary versus leap years. A calendar did more than count days: each dynasty issued a new calendar as a way of settling the order of Heaven and Earth. The Shang Shu (Book of Documents) opens with King Yao charging astronomers to compute the Sun, Moon, stars, and seasons.

Two systems still leave their mark: the Taichu calendar brought the 24 solar terms into the court calendar and treated a month with no Mid-Term as the leap month; the Shixian calendar switched to true-sun terms — solar terms fixed to the Sun’s true position, not equal divisions. The Vietnamese calendar follows that branch, but is anchored to UTC+7 rather than UTC+8.

A one-hour difference from the Chinese calendar can shift the day of the new moon — so Lunar New Year in the two countries sometimes falls a day apart (as in 2007, and expected in 2030). The app calculates in UTC+7.

Yin sets the month, Yang sets the year

One lunar cycle is about 29.53 days → one lunar month. One Earth orbit around the Sun is about 365.24 days → one year, anchored by the 24 solar terms. The 1st is the new moon; the 15th is the full moon. The winter solstice anchors lunar month 11, so the calendar does not drift from the seasons.

Twelve lunar months ≈ 354 days, about 11 days short of a solar year. The old rule: 7 leaps in 19 years (the Metonic, or Zhang, cycle). Any month that contains no Mid-Term is the leap month of the month before it. Under true-sun terms, the gaps between Mid-Terms are uneven — leaps often fall in lunar months 3–7, rarely 11–12.

Twenty-four solar terms — the solar rhythm

The solar terms divide the ecliptic into 24 segments of 15° each. Twelve Major Terms (the bamboo nodes, the cuts) alternate with twelve Mid-Terms (the mid-breaths). Mid-Terms are the calendar’s true anchors: each lunar month normally must contain one. The first month is anchored to Yushui; month 11 to Dongzhi.

Solar terms belong to the Yang side — their dates are nearly fixed on the solar calendar (Dongzhi ~22/12, Chunfen ~21/3) while they “jump” on the lunar calendar. Calling the whole system a “lunar calendar” is easy to misread: the year itself breathes with the Sun. The Calendar tab in the app shows the solar term of the day you are viewing.

Exact dates vary by ±1 depending on the year. The 12 Major Terms: Lichun, Jingzhe, Qingming, Lixia, Mangzhong, Xiaoshu, Liqiu, Bailu, Hanlu, Lidong, Daxue, Xiaohan. The 12 Mid-Terms: the remaining markers.

The Twelve Day Officers — the day’s duty officer

The Twelve Day Officers (the Establish–Remove cycle) date from Qin–Han. The day’s earthly branch is compared with the month’s branch (the month-establishing branch): a match is Establish, then in order Remove, Full, Balance, Stable, Initiate, Destruction, Danger (Nguy), Success, Receive, Open (Khai), Close — a twelve-day round. In places the app labels Receive as Collect (Thu); both are the same officer 收.

An old mnemonic: Establish, Full, Balance, Receive — Black; Remove, Danger, Stable, Initiate — Yellow; Success and Open may be used; Close and Destruction must not take on major work. That is only one layer. The Open officer can still be inauspicious if the mansion is hostile, the day is on the Black Path, auxiliary stars are heavy, or the day clashes with the native’s year. The app adds officer points to the Yellow–Black Paths and the 28 Lunar Mansions, then adjusts for the undertaking.

*Danger belongs to the Yellow Path group in the officer mnemonic, yet travel and groundbreaking are forbidden — auspicious for rites, not for every task.

The Twenty-Eight Lunar Mansions — inns of the Moon

The 28 Lunar Mansions are 28 markers on the celestial equator; the Moon lodges in one mansion each night (~27.3 days per circuit). Xiu 宿 means a resting place. The lacquer chest from the tomb of Marquis Yi of Zeng already depicts all 28 mansions. In date selection, the mansions take turns as the day’s officer — a 28-day cycle. The app gives each mansion a three-character set and an auspicious–inauspicious coefficient.

Four directions, four sacred beasts, seven mansions in each group. Auspicious–neutral–inauspicious is a separate layer and does not replace the 12 Day Officers. India has Nakshatra, the Arab world Manazil — the same lunar cycle, different name systems.

Auspicious, neutral, and inauspicious mansions

The grouping below is the usual shortcut for a quick date-selection reading — not an astronomical star table. A day with an auspicious mansion can still be unfavorable if the other layers run against it.

Yellow Path and Black Path

Twelve celestial spirits take turns ruling the day: six on the Yellow Path, six on the Black Path. The app places them by the day’s and month’s earthly branches; the sixth spirit is recorded as Bao Guang (some books call it Tian De — the same seat in the cycle of twelve). The Yellow Path in the almanac is not the astronomical ecliptic (the Sun’s apparent path, also called the Yellow Path).

A Yellow Path day is not enough to call a day good. The Day Officer, Lunar Mansion, auxiliary stars, and the householder’s year of age must still be weighed. The Calendar tab shows the day’s spirit; the Choose-date tab ranks a combined score.

Yellow Path hours — the accent within the day

The day is the ground; the hour is the point of start. Twelve Earthly Branch hours, two clock hours each. Six Yellow, six Black, shifting with the day’s branch. Common practice: first choose a day that matches the task, then a Yellow Path hour, then avoid an hour that clashes with the native’s year.

The calendar as a map of time

BaZi anchors the year and month pillars to solar terms — the destiny year changes at Lichun, not on the first day of the Lunar New Year. Zi Wei Dou Shu is placed by the lunar calendar and the hour. Xuan Kong divides the Nine Periods by solar terms. Qi Men changes its Bureau with each solar term. Plum Blossom (Mai Hoa) takes the heavenly stems and earthly branches of hour, day, month, and year.

Vietnamese life is anchored to the lunar side: Lunar New Year, Qingming, the Double Fifth Festival, the Mid-Autumn Festival, the Hùng Kings’ Commemoration, and family death-anniversary days. Major undertakings — marriage, opening a shop, groundbreaking, moving in — are still dated. The app does exactly that layer: the almanac plus date selection by task (betrothal and marriage, opening, groundbreaking, moving in, travel, contracts, burial, seeking wealth).

In this app

Calendar tab

Read each day: heavenly stems and earthly branches, Day Officer, Yellow–Black Path spirit, Lunar Mansion, solar term, Yellow Path hours, auxiliary stars. Free.

Choose-date tab

Filter by undertaking. The base score comes from Day Officer + Yellow–Black Path + 28 Lunar Mansions, then plus and minus for the selected task. The default list is free; advanced filters are rented by the hour.

Saved days

The home screen keeps marked dates — matching task, Day Officer, and spirit — so you can reopen them when needed.

Four common misunderstandings

It is not a purely lunar calendar

The Hijri calendar is purely lunar — Ramadan drifts through every season. The Vietnamese calendar has leap months and solar terms, so Lunar New Year always falls at the end of winter and the start of spring.

The 24 solar terms do not belong to the lunar calendar

They are anchored to the Sun. That is why Vietnam’s so-called “lunar calendar” is in fact a lunisolar calendar.

The Yellow Path is not an absolutely good day

It is one layer among four or five. A Yellow Path day that is also a Destruction officer day, or that clashes with the native’s year, is still unfavorable for the task at hand.

The Vietnamese calendar does not match the Chinese calendar day for day

Same principle, different meridians (105°E / UTC+7 versus 120°E / UTC+8). If the new moon shifts, the lunar month — and sometimes Lunar New Year — shifts by one day.

Quick questions

How do the lunar and solar calendars differ?

The solar (Gregorian) calendar follows the Sun alone, 365/366 days, convenient for administration. The East Asian lunisolar calendar: months follow the Moon, the year follows solar terms and leap months — tied to the agricultural year, festivals, and date selection.

Why does Lunar New Year fall on a different solar date each year?

Lunar New Year is the 1st of the first lunar month, the new moon nearest Lichun. Full 30-day months or a leap month make it slide within about 21/1–20/2 on the solar calendar.

Are the 12 Day Officers and the Yellow Path the same system?

No. Two independent cycles. A day that is both a favorable officer (Stable, Success, Open) and on the Yellow Path is what deserves to be called greatly auspicious. One side good and the other bad is only half-auspicious — still weigh the Lunar Mansion and the native’s year.

Do the 28 Lunar Mansions relate to the 12 zodiac signs?

They both divide the sky, from different roots. The mansions follow the celestial equator and the lunar cycle, and are used in date selection. The 12 zodiac signs follow the Sun’s path and come from Babylon–Greece.

How is a leap month calculated?

A lunar month that contains no Mid-Term. True-sun terms make the gaps between Mid-Terms uneven, so a month can “fall between” two Mid-Terms — that month keeps the previous month’s number plus a leap mark.

Must every task be dated?

Tradition reserves dating for major undertakings: marriage, building, opening a shop, burial, moving in, long journeys. Everyday tasks are not required. An auspicious day does not replace preparation.

Perpetual calendar, imperial calendar, lunisolar calendar — are they different?

The same system, different names. The perpetual calendar: a multi-year table of stems and branches and solar terms. The imperial calendar: the court calendar, with a date-selection section added. The app calls it the almanac — the full date-selection layers.