Adding Senjitsu

Getting the calendar type wrong (節月 vs 旧暦) is the most common and critical mistake. Many online sources are ambiguous. Always verify against Wikipedia's description and a published calendar.

Implementation Checklist

1. Add type definition in src/lib/calculations/senjitsu.ts

Add entry to SENJITSU_TYPES:

MY_SENJITSU: {
    name: "漢字名",
    reading: "ひらがな読み",
    romaji: "Romaji-name",
    meaning: "English description of meaning and significance.",
    meaningJa: "日本語での意味の説明。",
    isLucky: true, // or false
},

2. Add calculation logic in src/lib/calculations/senjitsu.ts

In the getSenjitsu() function, add the check. Follow existing patterns:

• If 節月-based: Use solarMonth (already computed) and dayBranchIndex/dayStemIndex from getDayZodiac()
• If 旧暦-based: Use oldDate from getOldCalendarDate() (already computed)
• Add lookup table as a module-level constant (e.g., const MY_TABLE: Record<number, number[]> = { ... })

3. Add i18n strings

• src/lib/i18n/ja.json - Add Japanese label if needed
• src/lib/i18n/en.json - Add English label if needed

4. Update the about/senjitsu page

• src/app/[lang]/about/senjitsu/ClientPage.tsx - Add explanation for the new 選日

5. Add icon support

• src/components/icons/SenjitsuIcon.tsx - Add icon/color for the new type

6. Write tests

• Add test cases in tests/senjitsu.spec.ts using known dates from a published calendar
• Test both Japanese and English display
• Include at least one positive and one negative test case

7. Verify

• Run npx playwright test tests/senjitsu.spec.ts to confirm tests pass
• Cross-check a few dates manually against a published calendar source

Common Mistakes

• Wrong calendar type: Using 節月 when 旧暦 is correct (or vice versa). Always verify on Wikipedia.
• Off-by-one in solar months: solarMonth is computed by comparing the date against getStarDate(). Month 1 = 小寒〜立春 (roughly Jan), Month 2 = 立春〜啓蟄 (roughly Feb), etc.
• Forgetting leap months: For 旧暦-based calculations, handle leap month cases (旧暦の閏月).
• Untested calculations: Always verify against a real published calendar before claiming correctness.