Lunisolar, lunar, and solar — three different calendar systems
People often say "lunar calendar" when they mean something more specific. There are three families. A pure solar calendar tracks the year by the sun's position — the Gregorian calendar in modern global use is one. A pure lunar calendar tracks the year by the moon's phases, with each month exactly one lunation; the Islamic calendar is the canonical example, and its dates drift backward by about 11 days each Gregorian year. A lunisolar calendar tries to do both: months follow the moon, but periodic adjustments keep the year roughly aligned with the sun.
The calendar this tool converts to is lunisolar. Months begin on the new moon, so each lunar month is 29 or 30 days. Twelve such months total 354 days — about 11 short of a solar year. Left alone, the seasons would drift through the calendar as in the Islamic system. Instead, an extra month (the leap month) is inserted seven times every nineteen years, and that pulls the calendar back into seasonal alignment.
Why a leap month appears every two or three years
The 19-year, 7-leap-month pattern comes from a coincidence the Greek astronomer Meton noticed in the 5th century BCE: nineteen solar years are almost exactly 235 lunations. Twelve months per year would be 228; the extra seven months distributed over the cycle make up the difference. East Asian astronomers worked out the same arithmetic independently, and the modern Korean and Chinese calendars still place leap months on this 19-year frame.
Which month gets the leap is determined astronomically, not by fixed rule. A leap month is inserted before any lunar month that does not contain a 中氣 (zhōngqì, the "mid-term" solar terms — every second one in the table below). The leap month takes the number of the preceding regular month and is marked 閏 (leap). So a year might run …4月, 5月, 閏5月, 6月…, with the leap fifth coming between the regular fifth and sixth. People born in a leap month celebrate their birthday on the corresponding regular month in non-leap years.
The 24 solar terms (二十四節氣)
The 24 solar terms split the ecliptic into 15° arcs. Each term begins when the sun reaches one of those longitudes, which means they are anchored to the sun, not the moon — their Gregorian dates barely vary year to year (within a day either way). Farmers used them to time planting and harvest, and they are still printed on calendars across the region. Equinoxes (春分 / 秋分) and solstices (夏至 / 冬至) are the four most familiar terms and align exactly with the Western definitions.
The table below uses Korean and Japanese readings alongside the Chinese characters. Dates are typical for the early 21st century and can shift ±1 day depending on the year.
| # | 漢字 | Korean | Japanese | Ecliptic longitude | Approx. Gregorian |
|---|
| 1 | 立春 | 입춘 | りっしゅん | 315° | 02-04 |
| 2 | 雨水 | 우수 | うすい | 330° | 02-19 |
| 3 | 驚蟄 | 경칩 | けいちつ | 345° | 03-05 |
| 4 | 春分 | 춘분 | しゅんぶん | 0° | 03-20 |
| 5 | 清明 | 청명 | せいめい | 15° | 04-05 |
| 6 | 穀雨 | 곡우 | こくう | 30° | 04-20 |
| 7 | 立夏 | 입하 | りっか | 45° | 05-05 |
| 8 | 小滿 | 소만 | しょうまん | 60° | 05-21 |
| 9 | 芒種 | 망종 | ぼうしゅ | 75° | 06-06 |
| 10 | 夏至 | 하지 | げし | 90° | 06-21 |
| 11 | 小暑 | 소서 | しょうしょ | 105° | 07-07 |
| 12 | 大暑 | 대서 | たいしょ | 120° | 07-22 |
| 13 | 立秋 | 입추 | りっしゅう | 135° | 08-07 |
| 14 | 處暑 | 처서 | しょしょ | 150° | 08-23 |
| 15 | 白露 | 백로 | はくろ | 165° | 09-07 |
| 16 | 秋分 | 추분 | しゅうぶん | 180° | 09-23 |
| 17 | 寒露 | 한로 | かんろ | 195° | 10-08 |
| 18 | 霜降 | 상강 | そうこう | 210° | 10-23 |
| 19 | 立冬 | 입동 | りっとう | 225° | 11-07 |
| 20 | 小雪 | 소설 | しょうせつ | 240° | 11-22 |
| 21 | 大雪 | 대설 | たいせつ | 255° | 12-07 |
| 22 | 冬至 | 동지 | とうじ | 270° | 12-22 |
| 23 | 小寒 | 소한 | しょうかん | 285° | 01-05 |
| 24 | 大寒 | 대한 | だいかん | 300° | 01-20 |
Korea, China, Vietnam, Japan: who still uses the lunar calendar
The four countries diverge sharply on day-to-day use. Korea keeps the lunar calendar for the major holidays — Lunar New Year (Seollal, 설날) and Chuseok (추석, the harvest moon festival) are public holidays calculated by the lunar date, as are individual ancestral memorial services (jesa, 제사). Day-to-day civil life runs on the Gregorian calendar, but lunar dates remain culturally load-bearing.
China uses the agricultural calendar (农历) for Lunar New Year (Chunjie, 春节), Mid-Autumn (Zhongqiu, 中秋), and Qingming. Vietnam follows essentially the same rules for Tết and runs its own version of the calendar with a different time zone, which occasionally shifts dates by a day relative to China. Japan switched to the Gregorian calendar on January 1, 1873 (Meiji 6) and now observes New Year on January 1; the old lunar holidays were re-anchored to fixed Gregorian dates or quietly dropped. The lunar terms still appear in seasonal poetry and tea-ceremony documents but no longer drive holiday timing.
Where lunar dates still matter day to day
For software with Korean or Chinese users, the lunar calendar still drives several real workflows. Birthdays for older relatives are often recorded by lunar date; in Korea, the elderly and the very traditional may not know their Gregorian birthday at all. Memorial services (제사 / 忌日) repeat every lunar year on the date of death — a calendar app missing the lunar conversion will silently break the most culturally important reminder of the year for many families.
Lunar dates also appear in wedding date selection (some couples consult almanacs that combine lunar dates with sexagenary year compatibility), the doljabi 100-day and first-birthday counts for infants in Korea, the timing of jeryebok (제례복) rituals, and traditional age (kor: 만 vs 띠 vs 세는나이) reckoning. Calendar apps marketed in Korea (Naver Calendar, Kakao Calendar) show both dates side by side; serious omission counts as a localization bug, not a missing feature.
How to use
Pick a **direction** — `Solar → Lunar` (default) or `Lunar → Solar` — then enter the **year / month / day** of the input date. For lunar inputs, you can also tick the **leap-month (윤달 / 閏月)** checkbox when applicable: leap months occur approximately every 2-3 years in the East Asian lunisolar calendar (7 leap months every 19 years, the **Metonic cycle**). The tool returns the converted date plus the **60-cycle stem-branch (六十干支)** for the lunar year and the weekday for the solar date. Date range supported is roughly 1900-2100; outside this range, accuracy decreases because the underlying astronomical data thins out.
The **East Asian lunisolar calendar** is *not* a pure lunar calendar (like the Islamic Hijri) nor a pure solar calendar (like the Gregorian). It tracks **both** the moon (each month begins at the new moon, so months are 29 or 30 days) and the sun (the year aligns approximately to the tropical year via inserted leap months). This dual-tracking is what makes the conversion non-trivial — you cannot compute lunar dates by simple arithmetic, you need a lookup table of the astronomical observations used by the **Korean Astronomy and Space Science Institute (한국천문연구원, KASI)** for Korean 음력 or the equivalent Chinese authority for 农历. This tool uses the **lunar-javascript** library which embeds these tables; the library is ~120 KB and is dynamic-loaded so the rest of the catalog stays small. Korean and Chinese lunar calendars are *almost* identical because both descend from the Han-dynasty astronomical standard, but rare edge-case dates near month boundaries can differ by 1 day depending on the observation longitude (Korean uses 135°E, Chinese uses 120°E — about 1 hour of solar time).
Examples
Korean Lunar New Year (설날) for 2026
Input
direction: Lunar → Solar
lunar: 음력 2026년 1월 1일 (정확히 설날)
leap: no
Output
Solar: 2026-02-17 (Tuesday)
Weekday: 화요일
Ganzhi: 丙午年 (병오년 / Fire Horse Year)
Animal: 🐎 Horse
Related lunar holidays from this conversion:
설날 연휴 — 2026-02-16 (음력 12/30 of 2025), 02-17 (음력 1/1), 02-18 (음력 1/2)
The 설날 (Korean Lunar New Year) **falls on the second new moon after the winter solstice**, which is why the Gregorian date shifts every year: 2024 was Feb 10, 2025 was Jan 29, 2026 is Feb 17. The earliest possible date is Jan 21 (next will be 2042), the latest Feb 21 (next 2034). Korea's **public holiday includes the day before and the day after the lunar new year**, making it a 3-day stretch on the Gregorian calendar — and when this 3-day stretch lands such that one day falls on a Sunday or other holiday, the **대체공휴일** rule extends it further. The 2026 calendar luckily lands cleanly on Mon-Tue-Wed (Feb 16-17-18), so no substitute is needed. The Chinese 春节 (Spring Festival) is the *same astronomical event* but observed in mainland China with up to 7 paid days off; the date converts identically to the second.
Leap month example — 2025 윤6월
Input
direction: Lunar → Solar
lunar: 음력 2025년 6월 15일
leap month: yes (윤달 6월)
Output
Solar: 2025-08-08 (Friday)
Weekday: 금요일
Ganzhi: 乙巳年 (을사년 / Wood Snake Year)
Note: 2025 has a leap 6th month — 음력 6월 15일 (regular) vs 윤6월 15일 (leap)
are *different* solar dates. Always check the 윤달 checkbox
when entering ambiguous lunar dates around mid-2025.**Leap months (윤달 / 閏月)** are the East Asian lunisolar calendar's mechanism to keep the lunar months aligned with the solar year. The lunar year (12 lunar months ≈ 354 days) is 11 days shorter than the solar year (365.24 days); without correction, lunar dates would drift through the seasons (like Islamic Ramadan does). The solution is to insert a 13th "leap month" approximately every 2-3 years. **The 19-year Metonic cycle** has 7 leap months: 19 × 12 + 7 = 235 lunar months ≈ 19 solar years. Which month gets duplicated depends on the **24 jieqi (節氣) solar terms** — a month without a "zhongqi" (middle solar term) becomes leap. Practical implication: lunar dates like "음력 6월 15일" can be ambiguous in leap years — you must specify whether you mean the *regular* 6월 or the *leap* 윤6월, which are typically ~30 days apart in the solar calendar. **Korean traditional birthdays** sometimes face this confusion when a family-recorded lunar date falls in a leap-month year.
Tracking Korean ages — lunar vs. Gregorian birthday
Input
scenario: convert a child's lunar birthday to the Gregorian one
for school registration
lunar: 음력 2020년 8월 15일 (추석에 출생)
lookup: what is the Gregorian date?Output
Solar: 2020-10-01 (Thursday)
Weekday: 목요일
Ganzhi: 庚子年 (경자년 / Metal Rat Year)
Family-tracking notes:
- Korean schools register by Gregorian date (2020-10-01)
- Family birthday celebration may track the lunar (chuseok-aligned each year)
- 2021 will celebrate on 2021-09-21 (lunar 8/15 in 2021)
- 2022 will celebrate on 2022-09-10 (lunar 8/15 in 2022)
→ the Gregorian birthday drifts ±20 days year over year
Legal age (2023+ uniformity reform): always Gregorian birthday
Korean families historically tracked birthdays on the **lunar calendar**, which means the Gregorian celebration date shifts year-over-year. A child born on 음력 추석 will have their lunar birthday celebrated on different Gregorian dates each year — sometimes early September, sometimes late October. This was the norm for grandparents' generations and remains common in rural areas and traditional families. Urban / younger generations increasingly track Gregorian birthdays exclusively. After the **2023 한국 나이 통일법** (Korean age uniformity reform) all *legal* ages are computed from Gregorian birthday only — but the social custom of lunar birthday celebrations persists in families with strong tradition. Korean astrology, fortune-telling, and 사주명리 readings all use the lunar birth date with the time of day; converting accurately to lunar requires the exact birth time *and* the geographical location to determine the local sunset boundary.
FAQ
Why are there leap months in the lunar calendar?
To keep the calendar **in sync with the seasons**. A pure lunar year (12 lunar months × ~29.53 days = 354.36 days) is **about 11 days shorter than a solar year** (365.24 days). Without correction, the lunar new year would drift earlier into the previous solar year by ~11 days annually — after 3 years, that's a full month adrift; after 33 years, the lunar calendar would be a full season out of phase. The leap-month system, formalized in the **Han dynasty (206 BCE - 220 CE)** but refined many times since, inserts a 13th lunar month approximately every 2-3 years. The **Metonic cycle** (named after Greek astronomer Meton, ~432 BCE) is the underlying math: **235 lunar months very closely equal 19 solar years**, with the 7 extra "leap months" distributed across that 19-year span. Astronomy-based calendars in Babylonia, Greece, and East Asia independently converged on this 19-year cycle because the math is convenient and the integer ratio is close to exact (off by ~2 hours per cycle).
How accurate is the conversion — is it byte-identical to KASI?
**Within 1 day for the 1900-2100 range, byte-identical for the central decades.** The underlying lunar-javascript library compiles its tables from authoritative astronomical sources (Korean KASI, Chinese 紫金山 Observatory, Japanese 国立天文台). For dates between ~1950 and 2050, the conversion is byte-identical to KASI's published tables. Outside this central range, there are rare 1-day discrepancies that arise from: (1) **observation longitude** — KASI uses Seoul (135°E) while Chinese 农历 uses 120°E, so a new moon falling near midnight Seoul time may fall on a different calendar day in Beijing reckoning; (2) **rounding of astronomical calculations** — the library's algorithms approximate where KASI uses the full ephemeris; (3) **historical edits** to the tables — Korean officials made minor corrections to pre-1900 lunar dates as recently as 2018. For legally binding dates (court filings, contracts, official birthdates), cross-reference KASI's official **음력 ↔ 양력 변환 서비스** at astro.kasi.re.kr.
How does the Korean lunar calendar differ from the Chinese one?
They are **nearly identical with rare 1-day differences** at month boundaries. Both descend from the **時憲曆 (Shixian calendar)** adopted in China in 1645 and brought to Korea shortly after — a Jesuit-influenced reform that introduced rigorous astronomical observation. The substantive difference today is **observation longitude**: Korean 음력 is computed using Seoul standard time (UTC+9, longitude 135°E), Chinese 农历 uses Beijing time (UTC+8, longitude 120°E). When a new moon falls near midnight Seoul time, it can be the **previous calendar day** in Beijing, which shifts every following lunar date by 1 day until the next new moon resync. These boundary cases happen maybe 1-2 times per decade. For 추석 (Korean lunar Aug 15) and 中秋节 (Chinese lunar 8/15), the dates are identical >95% of the time but can differ by 1 day in rare years. Vietnamese Tết and Korean 설날 use the same lunar calendar so dates match exactly; Japanese 旧暦 was abolished as the official calendar in 1873 but folk traditions still observe it on the Chinese/Korean schedule.
When are leap months in the next 10 years?
The **2024-2033 leap-month schedule** (Korean / Chinese reckoning, which agree on the leap-month placement except in rare boundary years): **2025** has 윤6월 (Jun 25 - Jul 24 solar approximately); **2028** has 윤5월; **2031** has 윤3월; **2033** has 윤11월. **Years without leap months**: 2024, 2026, 2027, 2029, 2030, 2032. The placement depends on which month "lacks a 중기 (middle solar term)" — different years have different leap-month positions, ranging from 윤1월 through 윤12월 (with 윤1월 and 윤12월 being rare). The 2033 윤11월 is unusual because end-of-year leap months are rare; the next such case will not occur until ~2052. For practical scheduling: if you have a lunar-calendar event near the leap month (e.g. a traditional anniversary on lunar 6월), check whether you mean the "regular" or "leap" 6월 carefully — they can be ~30 days apart.
What is the relationship between the lunar calendar and 24 节气?
The **24 节气 (jieqi / 절기 / sekki)** are a **solar-tracking system** layered on top of the lunisolar calendar, dividing the solar year into 24 equal-angle segments of 15° (the Earth's orbital position around the sun). Each segment is about 15 days, and they together describe seasonal phenomena: 立春 (start of spring, ~Feb 4), 春分 (spring equinox, ~Mar 20), 立夏 (start of summer, ~May 5), 夏至 (summer solstice, ~Jun 21), and so on. The 24 jieqi are **solar dates** with Gregorian dates that shift by only ±1 day year-over-year — they are *not* lunar at all. The connection to the lunisolar calendar is: **the placement of leap months is determined by which lunar month lacks a "zhongqi" (middle solar term)** — every other jieqi is a "zhongqi", and the lunar month containing no zhongqi becomes the leap month. This is why leap month placement varies year to year: the moon-phases shift relative to the solar terms each year, and roughly every 2-3 years one lunar month falls entirely between two zhongqi positions. The 24 jieqi are also used to identify the **traditional Asian agricultural calendar** — when to plant, harvest, prune — and persist in modern Korean / Japanese / Chinese culture as seasonal references.
How do I use this for traditional astrology or fortune-telling?
For **사주명리 (Korean four-pillar astrology) / 八字 (Chinese) / 算命 (Japanese)** the lunar conversion is one input among several. The four pillars are **year, month, day, hour of birth** each expressed as a 60-cycle stem-branch pair (干支), giving 4 × 60 = 240 nominal combinations — though actual viability is constrained by the calendar mapping. The **year pillar** is what this tool returns directly; getting the **month / day / hour pillars** requires additional astronomical calculations because they depend on the **solar-term boundaries** rather than calendar month boundaries (the month pillar changes at 立春 / 节气 transitions, not at lunar 1st). For accurate fortune-telling input you need: (1) Gregorian date and time of birth, (2) latitude/longitude of birth (for sunset boundary), (3) conversion to all 4 pillars via a 算命 tool that knows the jieqi tables. This converter only provides the year pillar; for the full 4-pillar reading you should use a specialized 사주 calculator app. As noted in our **`zodiac`** sister tool: empirical studies have not validated these systems for predictive use, but they remain culturally significant and historically rich.
Related concepts
The East Asian lunisolar calendar represents one of humanity's longest-running **astronomical observation programs**. Continuous record-keeping in China dates to the **Shang dynasty oracle bones (1300 BCE)**, which already used a 10-stem / 12-branch combinatorial day-counting scheme. The calendar was refined dozens of times across dynasties; the modern form descends from the **時憲曆 (1645)**, a Jesuit-collaboration reform that introduced rigorous astronomical observation methods. Korea adopted the 時憲曆 shortly after (formal adoption 1653) and maintained it as the official civilian calendar until the **갑오개혁 (1894)** transition to the Gregorian for state affairs; the lunar calendar has remained the cultural reference for traditional holidays, ancestor rites, and personal milestones to the present. Japan formally abolished the **旧暦** in 1873 as part of Meiji modernization, but the calendar persists informally in older agricultural communities, religious observances, and traditional festivals.
Three **conceptually adjacent calendar systems** are worth understanding in context. The **Islamic Hijri calendar** is purely lunar (12 lunar months ≈ 354 days) with no leap-month correction, so it drifts ~11 days earlier each year relative to the solar seasons — Ramadan moves from summer to winter and back over a 33-year cycle. The **Persian (Iranian) calendar** is purely solar but with precise astronomical observation that makes it more accurate than Gregorian over millennia. The **Hebrew calendar** is also lunisolar with leap-month corrections following a 19-year cycle (the same Metonic cycle), making it structurally similar to East Asian lunisolar but with religious-tradition-driven rather than imperial-bureaucracy-driven historical development. **Modern atomic time (UTC)** is a separate concept from any calendar — it counts seconds since 1972-01-01 with leap seconds occasionally inserted to track Earth's rotation; calendar systems sit on top of UTC for human-facing date representation.
Three adjacent **cultural and operational concepts** intersect with lunar-solar conversion. The **농경 (agricultural) calendar** in Korea historically anchored planting and harvest to 24 jieqi; modern Korean farmers still use jieqi for crop timing more than calendar dates because the solar terms track actual sun-Earth geometry. **Traditional birthdays vs. legal birthdays** — Korean grandparents typically remember their lunar birthday and convert each year; the 2023 한국 나이 통일법 made Gregorian birthdays the sole legal age reference but did not displace cultural practice. **Korean ancestor rites (제사 / 차례)** are scheduled by lunar date for the death anniversary of each ancestor, requiring a fresh annual conversion — most Korean families with elderly members maintain a small notebook of lunar-anniversary Gregorian conversions for several family ancestors. Our **`zodiac`** sister tool covers the related 60-cycle stem-branch system; the **`japanese-era`** tool covers Japan's era-name overlay on top of the Gregorian calendar; the **`holiday-calendar`** tool aggregates official public holiday lists across 200+ countries including lunar holidays like 설날 and 추석.
