Roughly once every two to three years, the lunar calendar inserts an extra thirteenth month to stay in sync with the seasons. If your birthday (or a parent's, or a grandparent's) falls in one of those months, you have probably run into confusing or contradictory answers about your "real" birthday, your zodiac animal, or your Saju chart. Here is what a lunar leap month actually is, which years have had one, and why it changes almost nothing about how your Saju is calculated.
A lunar month follows the Moon's cycle, about 29.5 days, so twelve lunar months add up to roughly 354 days — about eleven days short of the 365.24-day solar year. Left uncorrected, the lunar calendar would drift through the seasons over a matter of years, the way the Islamic calendar does. To prevent that drift, East Asian lunisolar calendars (Korean, Chinese, Vietnamese) insert an extra month about seven times every 19 years. That inserted month is called Yundal (윤달) in Korean, and it repeats the number of the month it follows: a leap month inserted after the sixth month is called "leap sixth month," not a thirteenth month with its own number.
Whether a given year gets a leap month, and where it falls, is not arbitrary. It is derived from the actual astronomical positions of the sun and moon, which is exactly why a general-purpose date library cannot answer "was I born in a leap month" correctly without a real lunisolar conversion table behind it.
We ran Cheonmyeongdang's own solar-to-lunar conversion engine (the same table it uses to build your Saju chart) across every year from 2015 to 2032 to find exactly where each leap month falls. This is computed directly from the engine's lunar data table, not copied from a generic calendar site:
| Lunar year | Leap month | First Gregorian day of the leap month |
|---|---|---|
| 2017 | Leap 5th month | 2017-06-24 |
| 2020 | Leap 4th month | 2020-05-23 |
| 2023 | Leap 2nd month | 2023-03-22 |
| 2025 | Leap 6th month | 2025-07-25 |
| 2028 | Leap 5th month | 2028-06-23 |
| 2031 | Leap 3rd month | 2031-04-22 |
No other year in that range (2015–2032) has a leap month. Notice the spacing is irregular — three years, then three, then two, then three, then three — because it tracks the actual gap between the lunar and solar calendars, not a fixed rule.
Here is the part almost every generic lunar-calendar explainer gets wrong when the topic turns to Saju or Bazi: your Saju chart's Month Pillar is not set by the lunar month at all, leap or otherwise. It is set by the 24 solar terms (Jieqi), a calendar built from the sun's exact ecliptic longitude, completely independent of the moon's phase. The boundary between one Saju month and the next is a solar-term crossing (for example, Ipchun around February 4 for the first month), not the first day of a lunar month.
That means a leap month changes your recorded lunar birthday and can change which zodiac stories or lunar folk holidays apply to you, but it does not create a thirteenth possible Month Pillar or a special edge case in the Saju calculation itself. Once your birth is converted to a solar-term position, a leap-month birth and a regular-month birth are treated by exactly the same deterministic rule. Cheonmyeongdang's engine still records the isLeapMonth flag internally (so your lunar date displays correctly), but that flag plays no role in which Month Pillar you receive.
One caveat that is not a contradiction: the leap flag still matters at the moment you enter a lunar birthday. If you type a leap-month date without marking it as a leap month, the calculator converts it to the wrong solar date, and a wrong solar date can move your Day Pillar and even your Month Pillar. The rule on this page applies after the date is converted correctly. For the input step itself, see Do I use my lunar or solar birthday?
No. The 12-animal zodiac cycle is assigned per lunar year, and a leap month is inserted inside a year, not between years. If you were born in the leap sixth month of 2025 (a Snake year), you are a Snake — the same as anyone else born earlier or later in that lunar year. The only situation where your zodiac animal genuinely becomes ambiguous is being born within a few weeks of the lunar new year itself, which is a separate boundary question from the leap-month question this article covers.
The lunar calendar year (354 days) runs about 11 days shorter than the solar year (365.24 days), so roughly every two to three years an extra month is inserted to keep the two aligned. That inserted month repeats the number of the month before it and is marked as a leap month, called Yundal in Korean. In our engine's lunar tables, years 2017, 2020, 2023, 2025, 2028, and 2031 each contain one.
No. Your zodiac animal is fixed by which lunar year you were born in, and a leap month is still part of that same lunar year. Being born in the leap sixth month of 2025, for example, does not shift you into 2026's animal. The animal only changes at the lunar new year boundary, not at the leap-month insertion point.
No, and this is the most common misconception. Saju's Month Pillar is set by the 24 solar terms (based on the sun's position), not by lunar months. A leap month is purely a lunar-calendar bookkeeping device to keep months aligned with seasons; it has no equivalent boundary in the solar-term calendar. So your Month Pillar is calculated the same deterministic way whether your lunar birthday falls in a regular month or a leap month, once your solar (Gregorian) birth date is known.
If you only know your lunar birthday from a birth record or a family account, check whether it was recorded with a leap-month marker. If you know your solar (Gregorian) birth date, a lunar conversion tool with leap-month support will flag it automatically. Cheonmyeongdang's calculator takes your solar birth date directly and detects the leap-month flag internally, so you never have to determine it yourself.