How the Chinese Lunar Calendar Actually Works

The Chinese calendar most people mean when they say “lunar calendar” is really lunisolar: its months are set by the moon and its years are kept in step with the sun by inserting an extra leap month roughly every three years. Taiwan uses it today only for festivals, temple dates and traditional birthdays — not for anything official.

Two clocks, not one

A purely lunar calendar, like the Islamic Hijri calendar, tracks only the moon and lets its year drift against the seasons — its New Year cycles slowly through all four seasons over about 33 years. The Chinese calendar does not do this. It tracks the moon for its months but anchors its year to the sun, so Lunar New Year always falls in the same rough window (21 January to 20 February) and festivals like Mid-Autumn always land in early autumn. That dual anchoring is what “lunisolar” means, and it is the single most useful fact for understanding everything else on this site.

How a month is defined

Each lunar month begins on the day of a new moon — the astronomical moment the moon sits between the earth and sun and shows no illuminated face from earth, not the first visible crescent. A new moon to new moon cycle (a synodic month) averages about 29.53 days, so lunar months alternate between 29 and 30 days. There is no fixed pattern of which; it is calculated from the actual position of the moon for the meridian used (historically Beijing, 120°E), which is why the calendar has to be computed rather than counted on fingers.

Twelve of these lunar months add up to about 354 days — roughly eleven days short of the 365.25-day solar year. Left alone, that gap would push Lunar New Year backwards through the seasons by about a month every three years, exactly as happens with the Islamic calendar.

How the year is kept in step: leap months

To close the eleven-day gap, the calendar inserts a whole extra month — a leap month (閏月) — about every two to three years, seven times in every 19-year cycle. A leap month is not a new, thirteenth kind of month; it repeats the number of the month before it (a leap month after the fifth month is written “leap month 5”) and is skipped in most counting, including most people’s birthdays. See leap months explained for how the calendar decides which month gets doubled.

The 24 solar terms

Running alongside the lunar months is a completely solar system: 24 solar terms (節氣) that divide the sun’s yearly path into equal 15-degree segments, each marking a seasonal milestone — Spring Commences, Qingming (Bright and Clear), the summer and winter solstices, and so on. These fall on near-fixed Gregorian dates every year (Qingming is always 4th or 5th April) because they are purely solar, which is also how a lunar-calendar leap month gets decided: a lunar month that contains no “principal term” becomes the leap month.

What sets Lunar New Year specifically

Lunar New Year is the first day of the first lunar month, and the first lunar month is fixed by counting forward from the winter solstice: it is the second new moon after the solstice (occasionally the third, when a leap month falls in between). That single rule is why the date moves around so much from year to year — see when Lunar New Year falls for the dates through 2035.

What Taiwan actually uses it for today

Taiwan’s civil calendar is Gregorian, dated in ROC (Minguo) years. The lunar calendar survives alongside it for a specific set of purposes: fixing the dates of traditional festivals (Lunar New Year, Dragon Boat, Mid-Autumn and more), setting temple worship days, choosing auspicious dates for weddings and moves, and recording birthdays the traditional way. Almost every printed Taiwanese calendar still shows both systems side by side for exactly this reason.

Frequently asked questions

Is the Chinese calendar lunar or solar?

Neither on its own — it is lunisolar. Months are set by the moon’s phases, but a leap month is inserted every two to three years to keep the year matched to the sun, so festivals stay in the same season every year.

Why do lunar months have 29 or 30 days?

A full cycle of the moon’s phases (new moon to new moon) averages 29.53 days, so calculated lunar months alternate between 29 and 30 days depending on the moon’s actual position, not a fixed rule.

What is a leap month?

An extra lunar month inserted about seven times every 19 years to stop the lunar year drifting against the seasons. It repeats the number of the preceding month and is explained in full on the leap month page.

How is Lunar New Year’s date decided?

It is the first day of the first lunar month, which is set as the second new moon after the winter solstice. Because new moon timing shifts, the date moves within a window of 21 January to 20 February.

Does Taiwan still use the lunar calendar officially?

No. Taiwan’s official civil calendar is the Gregorian calendar, dated in ROC (Minguo) years. The lunar calendar is kept alongside it for festivals, temple dates and traditional birthdays only.