Nhật Nguyệt: Lunar Calendar

Nhật Nguyệt is a Vietnamese lunar calendar (lịch âm, lịch vạn niên) and a shared family date book, built on a sourced, checkable knowledge base. Death anniversaries (giỗ), lunar birthdays, wedding anniversaries, full moon days: one person adds the date and everyone in the group is reminded. Every number in the app opens to show how it was computed and its source.

A giỗ follows the lunar calendar, so its Gregorian date changes every year. Enter it once; the app works out the date each year and tells you ahead of time.

Reminders for the whole family
- Five kinds: memorial day, birthday, anniversary, festival, other. Lunar or Gregorian; leap lunar months are handled.
- Make a group for your household or your father's side and send the invite link. Everyone in it sees the shared dates and is reminded.
- Each reminder notifies you twice: ahead of time and on the day.
- Guide to the occasion: a reminder for a memorial day, a full moon day, Tết or a wedding lists what to prepare day by day, each item linked to a sourced article.
- Your own reminders need no account; groups need a Google sign-in. Each account can create 2 groups of up to 3 people and join any number it is invited to.

A lunar calendar that shows its working
- The app opens on today's page: Gregorian date, lunar date, stem and branch (can chi), solar term, and the almanac's auspicious hours (giờ hoàng đạo).
- The date converter works both ways, and its result states where it comes from: algorithm, rules, sources, each step.
- Dates are computed in Vietnam time (UTC+7) with Hồ Ngọc Đức's algorithm, checked against 2,474 NASA new moons and 4,824 Paris Observatory solar term instants, 1900 to 2100.
- A home screen widget shows the lunar date, even offline.

A sourced knowledge base
- More than 1,400 articles on festivals, customs and how the calendar works, with a dictionary and a knowledge map. Footnotes give the book and page.
- Knowledge is arranged by field, each with its level of evidence: the calendar is computed and cross-checked; festivals and customs are documented; Tử Vi, Bát Tự and the I Ching are one school's rules, not verified.
- The How Nhật Nguyệt works screen lays out three layers (sources, rules, tools), with 27 open datasets under a DOI and a public corrections log.
- In Vietnamese, English and Chinese.

Look-ups by field
- Festivals and customs: Vietnamese festivals by month, with where they are held.
- Dates in the traditional almanac: auspicious hours, days by occasion, wedding dates, first visitor of the year.
- House and measures: Lỗ Ban ruler, house-building years.
- Classical charts and texts: Tử Vi and Bát Tự charts, the 64 hexagrams of the I Ching, hexagram casting, age matching by folk custom, baby names.
- Modern systems: numerology, Tarot.
- Palm reading and face reading need a sign-in: 5 a month per account across the two.
Each result states which kind of knowledge it is and which book or school it follows. Outside calendar computation, results are one school's rules or folk belief, not verified, for reference only.

Nhật Nguyệt AI
- An AI assistant answers from Nhật Nguyệt's own published articles and puts source numbers in the answer; tap a number to see the article cited. Sign-in required.
- Asked about your family's dates, it reads the reminders and groups in your account, only when you ask, and changes nothing.
- The app asks for your consent first and labels the assistant as AI. Answers can be wrong; each has a report button.

Privacy, no ads
- The app has no ads.
- Your birth date stays on your phone; if you sign in, it is also saved to your account.
- Group members see only each other's display names and the dates set for that group. Content can be reported, members blocked.
- Speak instead of typing: text only, no audio recorded.
- Delete your account in the app (Me, Your details) or via nhatnguyet.org/en/xoa-tai-khoan.

On Wear OS: lunar date complication, tile and upcoming reminders.

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