Lunar Calendar Converter

Convert a solar date to the Korean, Chinese and Vietnamese lunisolar date and back, with the leap month handled in both directions and the sexagenary stem-branch year name and its animal alongside. The range is 1900-01-31 to 2099-12-31, which is what the underlying table covers.

The awkward part of a lunisolar calendar is that it is not computed, it is tabulated. Each month begins on the day of a new moon, so months run 29 or 30 days in no fixed pattern, and a thirteenth month is inserted often enough to keep the year in step with the seasons — 74 times in the two centuries covered here. A lunar year is therefore anywhere from 353 to 385 days long, and a leap month repeats the number of the month before it rather than getting a name of its own.

Because it is a table, its errors travel by being copied. The version that circulates in tutorials and libraries has three entries this one does not follow. For 2033 it places a leap 7th month where the astronomical calendar has a leap 11th — the 윤달 동짓달 — five months early, which renumbers every month across the 148 days from 2033-08-25 to 2034-01-19; the correction keeps the year at 384 days, so nothing after it shifts. For 1996 it gives months 5 through 8 as 29, 30, 29, 30 where both references followed here say 30, 29, 30, 29, and the visible consequence was public: Chuseok 1996 is on record as 27 September and the copied table made it a day late. For 2060 it swaps the third and fourth months the same way.

Where the Korean and Chinese calendars genuinely disagree, they disagree for a real reason rather than a copying one: the new moon is an instant, and an instant near midnight falls on different dates either side of the meridian. Those cases are differences between the two traditions, not bugs. Conversions here are checked against both a Chinese implementation and a KASI-derived one.