Đổi lịch âm dương
Quy đổi ngày dương lịch sang âm lịch và ngược lại, kèm can chi và thông tin tháng nhuận theo múi giờ Việt Nam.
How it works
The Vietnamese calendar is lunisolar: months follow the moon, years follow the sun. A month begins on the day containing the new moon, so months run 29 or 30 days according to the actual lunation rather than a fixed rule.
Twelve lunar months are about 354 days, eleven short of a solar year. Left uncorrected, Tết would drift into summer within a few years. The correction is a leap month: roughly every two or three years a year has thirteen months, one of which repeats.
Which month repeats is decided by the sun, not by convention. A lunar month containing none of the twelve principal solar terms becomes the leap month and takes the number of the month before it — hence "leap fifth month" rather than a thirteenth month.
What separates the Vietnamese calendar from the Chinese one is the time zone. Every lunar and solar position here is computed for UTC+7, where the Chinese calendar uses UTC+8. When a new moon falls in the hour between them, the two calendars start the month a day apart — and the whole month follows the discrepancy.
The sexagenary cycle (can chi) runs alongside all of this and is unaffected by leap months. Ten stems against twelve branches give sixty combinations. The day cycle counts continuously from the Julian day number, so it never breaks; the year cycle follows the lunar year, which is why someone born in January often belongs to the previous animal year.
The algorithm is Hồ Ngọc Đức’s, the one most Vietnamese perpetual calendars use. It computes astronomical positions rather than reading a lookup table, so it is not limited to a pre-tabulated range of years.
Formula
solar date → Julian day number → nearest new moon → lunar month, with the leap month identified by principal solar terms (all evaluated at UTC+7)
Day stem = (Julian day number + 9) mod 10; day branch = (JDN + 1) mod 12. The year’s stem-branch follows the lunar year, not the Gregorian one.
Worked example
17 February 2026 in the Gregorian calendar:
- Lunar date
- 1st day, 1st month, 2026
- Year stem-branch
- Bính Ngọ (Fire Horse)
- Day stem-branch
- Nhâm Tuất
- Significance
- The first day of Tết
The first day of the first lunar month is the day containing the first new moon after the solar term Lập xuân, evaluated in Vietnamese local time. Tết therefore moves between 21 January and 20 February each year.
Frequently asked questions
Why does the Vietnamese lunar calendar sometimes differ from the Chinese one?
Because they use different time zones — UTC+7 against UTC+8. When a new moon falls in the hour between them, the first day of the month lands a day apart and the whole month shifts with it. This has put Tết and Chinese New Year on different dates.
What is a leap month?
A repeated month that absorbs the eleven-day gap between twelve lunar months and a solar year. One occurs every two or three years and takes the number of the preceding month, so a leap fifth month follows the fifth month.
Why do lunar months vary between 29 and 30 days?
The lunation is about 29.53 days, not a whole number. Since a month starts on the day containing the new moon, the actual cycle decides the length rather than any alternating rule.
How do I find the Gregorian date of a lunar birthday?
Switch the direction to lunar-to-solar and enter the lunar day, month and the year you want. Because the two calendars drift against each other, the same lunar date falls on Gregorian dates up to a month apart from year to year.
Which zodiac animal applies to someone born in January or February?
The one for the lunar year, not the Gregorian year. Someone born on 10 February 2026 still belongs to Ất Tỵ, because Bính Ngọ does not begin until Tết on 17 February 2026. This is the most common mistake in looking up a zodiac sign.
What is the day stem-branch used for?
It underpins most Vietnamese date selection — auspicious days, favourable hours and clashing years all key off the day’s branch. The sixty-day cycle runs continuously and leap months do not interrupt it.
How far back can it convert?
In principle any year, since it computes positions rather than reading a table. But for distant centuries the accuracy of the solar and lunar models degrades, and the reference time zone has itself changed historically, so results before the twentieth century deserve a second source.