Core answer: The Chinese lunar calendar is lunisolar: months follow the moon (29 or 30 days, starting at each new moon) while years track the sun via 24 solar terms, with 7 leap months inserted every 19 years to keep them aligned. Lunar month 1 day 1 = Spring Festival. Converting a lunar birthday to Gregorian requires a lookup table — the two calendars drift 10–12 days apart each year.

How the calendar actually works

  1. Months by the moon: each month starts on the astronomical new moon (朔); months are 29 (小月) or 30 (大月) days — you can't predict next year's pattern without ephemeris data.
  2. Years by the sun: the solar term 冬至 (winter solstice) must fall in month 11; if 13 new moons fit between two winter solstices, a leap month is inserted — the first month without a major solar term (中气) becomes the leap month.
  3. The 19-year Metonic cycle: 19 solar years ≈ 235 lunar months, so your lunar and Gregorian birthdays nearly coincide every 19 years (off by 0–2 days).

Why conversion needs tables, not formulas

Month lengths depend on exact new-moon timing (± hours shift 29↔30), and leap-month placement follows the no-zhongqi rule — a formula would need astronomical computation. That's why every converter (including ours) embeds a data table covering 1900–2100.

Worked examples

Example 1 — Two birthdays a year. Someone born lunar 8/15/1990 (= Oct 3, 1990 Gregorian) may celebrate lunar 8/15 each year (e.g., Oct 6 in 2025) — which lands on a different Gregorian date annually.

Example 2 — The 19-year check. Born Jan 29, 2006 (lunar 1/1/2006)? In 2025, Jan 29 is ALSO lunar 1/1 — the 19-year Metonic alignment made your 19th birthday coincide on both calendars exactly.

Example 3 — Leap month birthdays. Born in leap June (闰六月) of 1987? The next leap Junes are 2017 and 2025 — "real" lunar birthdays arrive roughly once a decade; most leap-month-born people celebrate the regular month instead.

Example 4 — Lunar age vs actual age. Traditional 虚岁 counts you as 1 at birth and adds a year at each Spring Festival — a baby born on lunar 12/29 is 虚岁 2 three days later.

Common mistakes and myths

  1. "The lunar calendar is purely lunar" — purely lunar calendars (Islamic Hijri) drift through all seasons; China's is lunisolar and stays season-locked — that's why it's formally called 农历 (agricultural calendar).
  2. Assuming lunar month = 30 days — it's 29 or 30, varying irregularly; "这个月有三十吗" requires a lookup.
  3. Zodiac by Gregorian year — the zodiac year boundary is Spring Festival (or Lichun in some schools), never Jan 1.
  4. "Lunar birthdays are the same date minus 11" — only as a rough drift estimate; leap months break the pattern entirely.
  5. Believing lunar is unscientific — it encodes both moon phases AND solar terms; farmers planted by it for two millennia because it tracks the seasons precisely.