# Quizzman Calendar — About (Lõi lịch pháp)

- HTML: https://lab.quizzman.com/tool/about-calendar
- VI locale file: https://lab.quizzman.com/tool/about-calendar-vi
- Tool: https://lab.quizzman.com/tool/calendar-converter

## Definition

**Quizzman Calendar** is an astronomical calendar converter. Its **lunar core** (`lunar-core`) computes New Moon (Sóc), 24 solar terms (VSOP87), leap months, Can Chi, True Solar Time, and historical Vietnam timezones — separately for **UTC+7 (Vietnam)** and **UTC+8 (China)**.

## Accuracy

- Recommended high-confidence band: **~1800–2200**
- New Moon target error: **< ~1 minute** (Meeus / Hồ Ngọc Đức family)
- Internal benchmarks: **108/108** regression + **19/19** edge cases
- Deterministic astronomy — **not** LLM-generated dates

## Algorithms

1. Julian Day Number  
2. New Moon — Jean Meeus / Hồ Ngọc Đức  
3. VSOP87 (abridged) sun longitude → 24 jieqi every 15°  
4. Leap month = lunar month without major solar term (Zhongqi)  
5. Can Chi from JDN (J2000.0 = Mậu Ngọ verified)  
6. True Solar Time = clock + longitude correction + Equation of Time  
7. Historical VN IANA / North–South 1960–1975 rules  

## What it supports

- Solar ↔ lunar (VN primary; CN compare)
- Can Chi day/month/year/hour pillars
- 24 solar terms to the minute
- True Solar Time by city/longitude (60+ VN cities)
- Early Rat / Late Rat (Dạ Tý / Chính Tý)
- Foundation for BaZi, Zi Wei, and I-Ching Can Chi

## Classic timezone divergence

When New Moon falls near midnight, VN and CN calendars can differ by a day — sometimes propagating into leap months / Tết (e.g. Ất Sửu 1985, 25/6/2006).

## Related

- I-Ching about: https://lab.quizzman.com/tool/about-iching
- Wiki VSOP87: https://wiki.quizzman.com/wiki/thuat-toan-vsop87-tinh-tiet-khi-hoang-kinh-mat-troi
- Lab llms.txt: https://lab.quizzman.com/llms.txt

## Related wiki

- https://wiki.quizzman.com/wiki/quizzman-iching-lich-phap-thuat-toan-do-chinh-xac
- https://wiki.quizzman.com/wiki/quizzman-calendar-engine
- https://wiki.quizzman.com/wiki/thuat-toan-vsop87-tinh-tiet-khi-hoang-kinh-mat-troi
- https://wiki.quizzman.com/wiki/mui-gio-lich-su-viet-nam-tai-sao-lich-am-vn-va-trung-quoc-khac-nhau
- https://wiki.quizzman.com/wiki/can-chi-he-thong-dem-60-chu-ky-3000-nam-lich-su
- https://wiki.quizzman.com/wiki/cong-cu-quy-doi-lich-thien-van-am-duong-can-chi-tiet-khi
- https://wiki.quizzman.com/wiki/gioi-thieu-cong-cu-quizzman-kinh-dich-lich-phap-tu-vi-bat-tu-jck-font