How we check every date against the Hong Kong Observatory
Last checked · written by the Fengshui Zen team; figures calculated from the same checked data as the calendar pages
Every Chinese calendar date on this site, from Chinese New Year to the start of each lunar month and each solar term, comes from one data file. We build it by calculation and then check it against the official tables of the Hong Kong Observatory. This page explains both steps and lists every place where the two disagreed.
Step 1: calculation
New moons and solar terms are computed with astronomical algorithms (the open-source lunar-javascript library) in China Standard Time, UTC+8, the reference time of the Chinese calendar. From those moments the calendar rules give the start of each month, the leap months, Chinese New Year and the festivals. Solar-term times are kept to the minute.
Step 2: comparison with the Observatory
The Observatory publishes one table per year from 1901 to 2100, listing every Gregorian day with its lunar date and marking the solar terms. We download all 200 tables and compare them with our data day by day:
| Checked | Count |
|---|---|
| Days compared | 73,048 |
| Lunar month starts | 2,474 |
| Solar-term dates | 4,800 |
| Disagreements | 7 |
The 1901 to 2100 range has 73,049 days; the Observatory's 2069 file has no row for December 30, so that one day is not compared.
The 7 disagreements
Each one is a moment that falls within minutes of midnight Beijing time, where a tiny difference in how the Sun's or Moon's position is calculated decides the date. The Observatory notes that such cases can differ by a day. In all of them we use the Observatory's date and show no time.
| Date used | Event | Our calculation | Observatory |
|---|---|---|---|
| Nov 23, 1912 | Minor Snow (Xiaoxue) | Nov 22, 1912 | Nov 23, 1912 |
| Sep 24, 1913 | Autumn Equinox (Qiufen) | Sep 23, 1913 | Sep 24, 1913 |
| Dec 7, 1917 | Major Snow (Daxue) | Dec 8, 1917 | Dec 7, 1917 |
| Sep 8, 1927 | White Dew (Bailu) | Sep 9, 1927 | Sep 8, 1927 |
| Jun 21, 1928 | Summer Solstice (Xiazhi) | Jun 22, 1928 | Jun 21, 1928 |
| Jan 21, 1979 | Major Cold (Dahan) | Jan 20, 1979 | Jan 21, 1979 |
| Sep 28, 2057 | Start of a lunar month (new moon) | Sep 29, 2057 | Sep 28, 2057 |
What this does not cover
- Solar-term times to the minute: calculated, not checked, because the tables give dates only.
- 1900: calculated only; the tables start in 1901.
- Public holidays: governments announce them each year, so the festival pages describe usual practice.
If a date looks wrong, email fourtuneinsight@gmail.com with the page address and we will check it against the tables. More on the site and who runs it: About.
FAQ
Are the solar-term times on this site official?
The dates are checked against the Hong Kong Observatory. The times to the minute are our own calculation, because the Observatory tables give dates only. Near midnight that difference matters, which is why the 7 cases listed on this page show the Observatory's date and no time.
Why follow the Hong Kong Observatory and not your own calculation?
The Observatory is a government body that publishes conversion tables for every day from 1901 to 2100. When a moment falls within minutes of midnight, small differences between calculation methods can move it across midnight; an official table is the better reference.
Is 1900 checked too?
No. The Observatory's tables start in 1901, so the 1900 dates on this site are calculated only.
Data pages for this guide
More guides
This guide explains the traditional calendar; it is not a prediction or advice. How the dates are calculated.