Gregorian ↔ Chinese Lunar Date

Features

• Both directions: Gregorian → lunar and lunar → Gregorian (the lunar side accepts either the Chinese character form or plain digits)
• The lunar date in both Chinese characters and digits — characters for reading, digits for computing
• Leap months are labelled, and you must state them on input (a numeric form cannot distinguish them)
• The sexagenary cycle for year, month and day, plus the zodiac animal and star sign
• ⚠️ The star sign comes with its symbol, element (fire / earth / air / water) and date range — beyond “I am a Leo”, those are the two things people actually want
• ⚠️ The date ranges are the conventional fixed dates, not astronomical boundaries: the exact moment the sun enters each sign shifts by hours from year to year, so within a couple of days of a boundary the ephemeris is the authority (the result says so)
• With a time, the double-hour plus a note on the early/late rat hour distinction
• Solar terms and festivals are flagged when they fall on the day
• Days until the next lunar new year — often more useful for planning than the lunar date itself
• The lunar dataset is loaded on demand, so other pages are unaffected
• ⚠️ The lunar arithmetic is pinned to 28 known dates (Spring Festivals, Mid-Autumns and Dragon Boats from 1980–2027); the table is in the tests

How to use

1. Enter a Gregorian date (a time may follow, e.g. 2026-09-27 14:30)
2. The lunar date, sexagenary cycle, zodiac animal, star sign and solar term appear immediately
3. To go the other way, set the direction to lunar → Gregorian and enter the lunar date
4. Tick the leap-month box when the lunar month you entered is a leap month

FAQ

Why use a library for the lunar calendar instead of computing it?

Because the Chinese calendar is not a formula. It is an astronomical table: which months are long and which years have a leap month depends on where the new moon and the major solar terms (the sun’s apparent longitude reaching a multiple of 30°) fall, and both need high-precision astronomical series. An implementation was attempted, but the problem was having no reliable reference: a coefficient recalled wrongly that misplaces one year’s leap month still renders a perfectly ordinary lunar date — nothing errors, nothing looks odd. So this uses an established MIT-licensed library, pinned to 28 dates whose answers I know independently: 17 Spring Festivals, 6 Mid-Autumns and 5 Dragon Boats between 1980 and 2027, all matching.

Why must I specify the leap month myself?

Because a numeric form cannot say. `2026-08-17` could be the eighth month or the leap eighth month, and those can be a month apart on the Gregorian calendar. The Chinese character form has a place to write that marker; the numeric form does not, so it is read as a non-leap month. Tick the leap-month box rather than guessing, whose cost is being a month out.

Why does the rat hour straddle midnight? Which day is 23:30?

Because the day is divided into twelve two-hour blocks, so the rat hour necessarily spans midnight (23:00–01:00); that is inherent to the system. The complication is that in four-pillar numerology, a moment after 23:00 counts as the next day’s rat hour, while 00:00–01:00 counts as the current day’s. This tool gives the clock reading — 23:30 is the rat hour, on the same date — and notes the difference next to the result. For a four-pillar chart, use the other convention; the two are not the same thing.

Why does the sexagenary cycle differ from what I see elsewhere?

Because the boundaries for year and month are drawn differently. The year here follows the lunar year (starting at the first day of the first month), whereas four-pillar numerology uses the start of spring: someone born before it still carries the previous year’s stem-branch. Likewise the month cycle follows solar terms while lunar months follow new moons, so the two can differ by half a month. When comparing, check which boundary each side used — a one-character difference usually comes from that, not from an error.