Thai Lunar Calendar (泰國陰曆)

The Thai lunisolar calendar (ปฏิทินจันทรคติไทย) — the calendar the kingdom's Buddhist holy days still run on — computed by the classical SURIYAYATRA arithmetic of the Chulasakarat era, ported faithfully from the Eade & Gislén formalization: horakhun, kammacapon, avoman, masaken and tithi all flow from the 292,207-days-in-800-years constant, and from them the year's type — ปกติมาส (354 days), อธิกวาร (355, the extra day in month 7) or อธิกมาส (384, the doubled month 8, เดือนแปดหลัง) — including the four edge-rule adjustment passes where naive implementations drift. Any Gregorian date returns its Thai lunar date (เดือน + ขึ้น/แรม + ค่ำ), the Chulasakarat and Buddhist Era years, the year type with its working numbers, and whether the day is a WAN PHRA (the Buddhist sabbath: ขึ้น 8, ขึ้น 15, แรม 8, and the month's last waning night). VERIFIED against all nine official Bucha public holidays 2024–2026: Makha, Visakha and Asalha Bucha each compute to ขึ้น 15 ค่ำ of the canonical month, including 2026's leap-shifts (Makha in month 4, Asalha in the second month 8). DISCLOSED: this is the civil-arithmetic calendar the public holidays follow; observational practice can differ in edge years, and Songkran is a separate solar matter. Pure computation, no LLM.

Cost: 2 TIAN Points per call. tRPC procedure thaiLunar.date.