The Month Pillar: Solar Months and the Five Tigers Rule

The month pillar does not follow calendar months or lunar months. It follows the sun: a new month begins at each of the twelve jie solar terms, the branch of each month is fixed, and the stem is set by the year's stem through the five tigers rule.

Months measured by the sun

The 24 solar terms divide the sun's yearly path into steps of 15 degrees. They alternate between two kinds. The twelve jie (節) terms open a month; the twelve qi (氣) terms, such as the equinoxes and solstices, sit in the middle of a month and do not change the month pillar.

The first month starts at 立春, when the sun reaches 315 degrees of longitude, and each following month starts 30 degrees further on. The haru engine does exactly this: it takes the sun's longitude at the moment of birth, subtracts 315, and divides by 30 to find which of the twelve months it is. For example, 雨水 (Rain Water) fell on 19 February 2026 at 00:52 KST, but because it is a qi term, that day still belongs to the month that began at 立春.

The branches are fixed

Each month always carries the same branch. The month that begins at 立春 is always a 寅 (Tiger) month, the one at 驚蟄 is 卯, and so on around the circle to 丑, the month that starts at 小寒 in early January. The month holding the winter solstice is always 子. Because the branch is fixed, the only thing that changes from year to year is the stem.

The five tigers rule (五虎遁)

Twelve months a year and sixty pairs in the cycle means the month pairs repeat every five years. The stem of the first month, the 寅 month, depends on the year stem, and stems five apart share the same starting point. So a 甲 year and a 己 year both open with 丙寅, a 乙 or 庚 year with 戊寅, and so on. The rule takes its name from 寅, the Tiger, because it tells you which stem the Tiger month receives.

In the engine this becomes one line: month stem = (year stem mod 5 x 2 + 2 + month number) mod 10, with stems counted from 甲 = 0 and the months counted from the 寅 month = 0. For a 丙 year, the year stem is 2: 2 mod 5 = 2, times 2 is 4, plus 2 is 6, which is 庚. So the first month of a 丙 year is 庚寅, and each later month moves one stem forward.

五虎遁: month pillars by year stem, from the 寅 month to the 丑 month
Year stem	寅	卯	辰	巳	午	未	申	酉	戌	亥	子	丑
甲/己	丙寅	丁卯	戊辰	己巳	庚午	辛未	壬申	癸酉	甲戌	乙亥	丙子	丁丑
乙/庚	戊寅	己卯	庚辰	辛巳	壬午	癸未	甲申	乙酉	丙戌	丁亥	戊子	己丑
丙/辛	庚寅	辛卯	壬辰	癸巳	甲午	乙未	丙申	丁酉	戊戌	己亥	庚子	辛丑
丁/壬	壬寅	癸卯	甲辰	乙巳	丙午	丁未	戊申	己酉	庚戌	辛亥	壬子	癸丑
戊/癸	甲寅	乙卯	丙辰	丁巳	戊午	己未	庚申	辛酉	壬戌	癸亥	甲子	乙丑

The 2026 month boundaries

2026 is a 丙午 year, so it uses the 丙/辛 row above. The table below lists the twelve jie terms that open each month of the 2026 pillar year, with their exact times in Korean Standard Time. The last month, 辛丑, begins at 小寒 on 5 January 2027 and runs until the next 立春, on 4 February 2027 at 10:46 KST.

Month pillars of the 丙午 year (starts in Korean Standard Time)
Jie term	Starts (KST)	Branch	Month pillar
立春 Start of Spring	2026-02-04 05:02	寅	庚寅
驚蟄 Awakening of Insects	2026-03-05 22:59	卯	辛卯
清明 Clear and Bright	2026-04-05 03:40	辰	壬辰
立夏 Start of Summer	2026-05-05 20:49	巳	癸巳
芒種 Grain in Ear	2026-06-06 00:48	午	甲午
小暑 Minor Heat	2026-07-07 10:57	未	乙未
立秋 Start of Autumn	2026-08-07 20:43	申	丙申
白露 White Dew	2026-09-07 23:41	酉	丁酉
寒露 Cold Dew	2026-10-08 15:29	戌	戊戌
立冬 Start of Winter	2026-11-07 18:52	亥	己亥
大雪 Major Snow	2026-12-07 11:52	子	庚子
小寒 Minor Cold	2027-01-05 23:10	丑	辛丑

Checking it against real dates

The last example shows why the year matters: January belongs to the tail of the previous pillar year, so its month stem comes from the previous year's row. At 立春 both the year and the month change together, from 乙巳 and 己丑 to 丙午 and 庚寅.

21 June 2026: 芒種 began at 00:48 on 6 June, so the engine gives the month pillar 甲午.
25 September 2026: 白露 began at 23:41 on 7 September, so the month pillar is 丁酉.
22 December 2026: 大雪 began at 11:52 on 7 December, so the month pillar is 庚子.
1 January 2026: still in the 乙巳 year, before 立春. Using the 乙/庚 row, the 子 month is 戊子, which is what the engine gives.

Questions

Is the month pillar the same as the lunar month?

No. The month pillar follows the jie solar terms, which fall on nearly the same solar dates each year, around the 4th to 8th of each month. Lunar months move with the moon and can start weeks apart from them.

Why is it called the five tigers rule?

The first month of the pillar year is always a 寅, or Tiger, month. The rule gives the stem of that Tiger month for each of the five pairs of year stems, and the rest of the year follows in order.

What if I was born on the day of a jie term?

Then the exact time matters, because the month can change partway through the day. The haru engine measures how many hours a birth is from the nearest boundary term so that close cases can be noted.

Does a leap month in the lunar calendar affect the month pillar?

No. Leap months belong only to the lunar calendar. The month pillar is counted from the sun's position, so there are always exactly twelve months in a pillar year.