Bazi, the Chinese four-pillar chart, divides a life into ten-year Major Luck cycles, 大運 (dàyùn), the first beginning some time after birth. The classical rule turns the gap between birth and a solar term into that wait, three days to a year. We ran it over every minute of 2026, male and female charts alike. Each whole start age from 0 to 9 took about a tenth of the year's birth minutes. Only 1.87% started at ten or later, about one minute in 53, and none later than 10.474 years. The Ming-dynasty 三命通會, attributed to Wan Minying, states the rule and credits it to 古人, the ancients; the 珞琭子消息賦 in the 淵海子平 has it too. Quotations follow Wikisource's simplified text, shown in traditional.
Yang-year boys and yin-year girls wait for the next 節
Direction comes from the year stem's polarity and the chart's sex together. The chapter 論大運 in the Wikisource text of its second volume reads 「陽男陰女，大運以生日後未來節氣日時為數，順而行之；陰男陽女，大運以生日前過去節氣日時為數，逆而行之。」 A man born in a yang year or a woman born in a yin year counts forward to the next term, with the cycles progressing from the month pillar. A man born in a yin year or a woman in a yang year counts backward to the previous term.
The year turns at 立春 (lìchūn), the start of spring. In 2026 立春 fell at 04:02 on 4 February, UTC+8, so earlier births belong to the yin year 乙巳 and later ones to the yang year 丙午. A boy and a girl born together always stop at different 節.
Every minute of gap to the 節 adds two hours to the start age
The book shows its sums: 「凡三日有三十六時，乃見三百六十日為一歲之數」. Three days hold 36 two-hour 時辰, at ten days of life each a 360-day year. So 4,320 minutes of gap make a year, 360 a month and 12 a day, and each minute is two hours of life. A baby born an hour before its 節, in a forward chart, starts the first cycle at five days old.
The chapter's own case, a 甲子-year man born five days and three 時辰 before 立春, comes to 63 時辰, or 630 days: 「一歲奇九月之大運」, one year and nine months. Other clauses of this passage are garbled in the Wikisource copy and are not quoted.
Counting to a 中氣 caps the wait a little over five years
Solar terms alternate between 節, which open the bazi months, and mid-month 中氣. The 三命通會 writes 節氣 and never says 節 only; that reading is inferred from its arithmetic. It says 「在一月之中有三百六十時」: a month's 360 時辰 make ten years, and only a 節-to-節 month, about thirty days, is that long. Over 1901–2100 the longest gap between neighbouring terms is 15.736 days, 夏至 to 小暑 in 1904, which would hold every start to 5.245 years.
The Wikisource 淵海子平 glosses the rule as 「折除者乃一年二十四氣，七十二候」, naming all twenty-four terms. This describes the year's divisions without specifying which term ends the count, so it could be read as supporting either convention. The lunar-javascript library's documentation, on its author's 6tail.cn site, is explicit: a forward chart counts on to the next 節 and its hour.
Whole-year start ages up to nine take near-equal shares of the year
For each of 2026's 525,600 minutes we took the gap to the rule's 節, divided by 4,320 and kept elapsed whole years. Every minute counts once here, so the table describes the rule itself and says nothing about real births.
| Start age (whole years) | Male charts | Female charts |
|---|---|---|
| 0 | 9.04% | 10.68% |
| 1 | 9.34% | 10.32% |
| 2–7, each | 9.86% | 9.86% |
| 8 | 10.47% | 9.19% |
| 9 | 10.11% | 8.75% |
| 10 | 1.87% | 1.87% |
Within each of the year's twelve gaps, every whole year of age gets three days of birth minutes, so the middle rows are flat by construction at 51,840 minutes in all, or 9.86%. The edges move with the polarity flip at 立春 and the calendar year opening and closing mid-gap.
Ten or more needs a gap longer than thirty days. In 2026 those run from 驚蟄 through 立冬, March to November. The latest, 10.474 years, is a boy born just after 芒種 at 23:48 on 5 June. Over 1901–2100 the longest gap is 31.438 days, 芒種 to 小暑 in 1901, or 10.479 years.
Minute-level counting changes the start on nearly a fifth of charts
The library's getYun function documents two ways to compute the start, 起運, and uses method 1 by default. Method 1 counts whole days and 時辰, in steps of ten days of life; method 2 counts minutes. We compared them over 1,051,200 charts, every minute of 2026 for both sexes. Nearly one chart in five, 18.37%, got a different start year or month. A different whole year came up for 1.53%, 16,035 charts, and in every one of them method 1 gave the later start, by exactly one year.
Singapore's clock has also moved over the years. The IANA time zone database has Singapore at UTC+6:55:25 until June 1905, UTC+7 to 1932, UTC+7:20 from 1933 to August 1941, and UTC+7:30 from then until the end of 1981, apart from UTC+9 under occupation; the 1982 move to UTC+8 is covered in Singapore birth times and solar time. A 30-minute error moves the start by 2.5 days: a boy born at 06:00 on 6 March 1975 starts at 9 years 9 months 27 days on UTC+7:30, and two and a half days sooner on UTC+8.
Our major luck cycle calculator, as reworked on 7 November 2026, shows the gap-to-age arithmetic beside method 1's answer and lists the historical offsets. Until that rework it offered four offsets, described its count as running to the "nearest" 節, and took 虛歲, nominal age, from Gregorian years. The reworked version adds a year at each 正月初一, the lunar new year; over every whole hour of 2026 for both sexes, that changed 3,436 of 17,520 charts (19.61%). The calculator counts 虛歲 at 正月初一 and says so.
Without a birth hour the start age can be off by up to two months
Counted from noon, the true hour is at most 720 minutes off, two months of start age, and over 2026 the worst error was exactly 2.000 months while the same 節 was counted. The exception is a day on which a 節 falls, twelve of them in 2026 at minute resolution, when the hour picks the term. Born at noon on 5 June 2026, UTC+8, a boy counts 708 minutes to 芒種 and starts at 0 years 1 month 29 days, first cycle 甲午. At 23:50, he counts on to 小暑 and starts at 10 years 5 months 20 days, first cycle 乙未.
Children still waiting for a cycle take their reading from 小運
The 三命通會's section on 小運, a minor cycle moving one pillar a year, says 「童限未交大運，專用此法」: before a child enters a Major Luck cycle, this method alone applies. Once the cycles begin, the book weights the branch, 「蓋大運重地支」, and sets each cycle against the strength of the whole chart, 「更看四柱強弱何如」. On the book's account, then, the start age marks the change from one pillar a year to one pillar a decade.
