Free tool
Lunar calendar
A lunar date is derived from the true positions of the moon and sun, not from a fixed offset table. Vietnam computes on GMT+7 and China on GMT+8, so when a boundary falls near midnight the two calendars differ by exactly one day. The same birth date can therefore yield two different lunar dates depending on where you look it up.
Convert any solar date to the lunar calendar accurately for Vietnam's time zone, see the day and year's Stems-and-Branches, auspicious hours, and holidays in the month, free to use.
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What is the lunar calendar and how is it calculated?
The lunar calendar Vietnamese people use day to day, for death anniversaries, Lunar New Year (Tết), and the 1st/15th of each lunar month, is technically a lunisolar calendar: months follow the Moon's cycle (about 29.5 days) while the year is periodically corrected against the Sun's cycle with an added leap month, so Tết doesn't drift across the seasons the way a purely lunar calendar (like the Islamic calendar) does.
Technically, a lunar year has 12 months (353-355 days) or 13 months in a leap year (383-385 days), with roughly 7 leap years in every 19-year cycle (the Metonic cycle, also used independently by the old Chinese and Hebrew calendars). A lunar month starts on the New Moon day, when the Moon sits between Earth and the Sun, calculated today with modern astronomy rather than the naked-eye observation used in ancient times.
The lunisolar calendar Vietnam uses today traces back to ancient China's agricultural calendar, adapted and localized over centuries; since 1967 Vietnam has calculated it using the UTC+7 timezone (Hanoi time) instead of the previously used UTC+8 (Beijing time), so in some years Vietnam's and China's Tết fall a day apart.
Each lunar day is also assigned a Can-Chi pair (10 Heavenly Stems × 12 Earthly Branches, repeating on a 60-day cycle), an ancient day-counting system that underlies several other tools on this site, such as auspicious hours, zodiac compatibility, and Tử Vi charts. This tool converts a solar (Gregorian) date to its lunar date and Can-Chi using standard astronomical algorithms, matching the results printed on common wall/desk lunar calendars.
The 10 Heavenly Stems (Giáp, Ất, Bính, Đinh, Mậu, Kỷ, Canh, Tân, Nhâm, Quý) and 12 Earthly Branches (Tý, Sửu, Dần, Mão, Thìn, Tỵ, Ngọ, Mùi, Thân, Dậu, Tuất, Hợi) each cycle independently and combine to form 60 unique Can-Chi pairs before repeating, known as the "sexagenary cycle" (lục thập hoa giáp). This same 60-unit cycle is applied to count not just days but also years, months, and hours, which is why a birth year like "Giáp Tý" or "Ất Sửu" identifies a specific year within any given 60-year span.
Leap months don't get their own new name; a leap month simply repeats the number of the month before it (for example a leap 4th month, "tháng 4 nhuận", follows directly after the regular 4th month). Which month becomes the leap month in a given leap year follows a fixed astronomical rule, not an arbitrary choice, and this tool applies that same rule when converting dates so leap years are handled correctly.
Vietnam and China now largely share the same lunisolar calculation method, but because Vietnam uses Hanoi's UTC+7 timezone while China uses Beijing's UTC+8, the exact moment of a New Moon can fall just before midnight in one timezone and just after in the other. In years where this timing difference pushes the New Moon across a date boundary, Vietnam's Tết and China's Lunar New Year end up one day apart, a well-documented quirk rather than a calculation error.
| Group | Members |
|---|---|
| 10 Heavenly Stems | Giáp, Ất, Bính, Đinh, Mậu, Kỷ, Canh, Tân, Nhâm, Quý |
| 12 Earthly Branches | Tý, Sửu, Dần, Mão, Thìn, Tỵ, Ngọ, Mùi, Thân, Dậu, Tuất, Hợi |
| Can-Chi cycle | 10 Stems × 12 Branches combine into 60 unique pairs before repeating (the sexagenary cycle) |
Frequently asked questions
Is this lunar calendar conversion accurate?
The tool uses a standard lunisolar conversion algorithm based on modern astronomy, matching the results printed on common Vietnamese wall/desk calendars.
Why do some lunar years have a leap month?
Because 12 lunar months (about 354 days) run roughly 11 days shorter than a solar year (365 days); once that gap accumulates enough, the lunar calendar inserts a leap month (about 7 leap years in every 19-year cycle) so New Year doesn't drift across the seasons.
What is a birth date's Can-Chi pair used for?
The Can-Chi pair for your birth day/hour/year underlies several other tools on this site, such as auspicious hours, zodiac compatibility, and Tử Vi charts, since all of them build on the 60-unit Can-Chi cycle.
Is the Vietnamese lunar calendar the same as the Chinese one?
Nearly identical but not always: both use the same calculation method, but Vietnam uses the UTC+7 timezone while China uses UTC+8, so in some years the two countries' Tết fall a day apart.
Can this tool convert past and future dates?
Yes, enter any solar date (past or future) to get its corresponding lunar date and Can-Chi pair.
What are the Sóc and Vọng days?
Sóc is the 1st day of the lunar month (new moon), Vọng is the 15th (full moon), the two most important markers in a lunar month, commonly tied to ancestor worship and temple visits.
Why did Vietnam once use Beijing's timezone for its lunar calendar?
Before 1967, Vietnam calculated its lunar calendar using the UTC+8 (Beijing) timezone, a holdover from historically sharing China's calendar system; only from 1967 did it officially switch to UTC+7 (Hanoi time).