It converts a date from the solar (Gregorian) calendar to the Vietnamese lunar calendar, called âm lịch, and back. For any day from 1800 to 2199 it shows the lunar day, month and year, whether the month is a leap month (tháng nhuận), the can chi names of the day, month, year and first hour, the zodiac animal, the solar term (tiết khí) and how long the lunar month is. A month calendar prints the lunar day under every solar day, and a separate box gives the date of Tết (lunar 1/1) for any year.
The page opens on 10 February 2024, which is day 1 of month 1 of the year Giáp Thìn: Tết 2024. The day itself is also called Giáp Thìn, and the month Bính Dần.
How to use it
- Choose Solar to lunar and pick a date, or click any day in the calendar. Arrow keys move through the calendar one day or one week at a time, and the conversion follows.
- Choose Lunar to solar to go the other way: type the lunar day and year, pick the month, and tick Leap month only if you mean the repeated month. Mid-Autumn 2025 (15/8) gives Monday, 6 October 2025.
- Type a year in the Tết box to see the date of the lunar New Year. 2026 is Tết Bính Ngọ on 17 February 2026.
If a lunar date does not exist the tool says why instead of guessing: lunar year 2025 has no leap month 8 (its leap month is the 6th), and a month with 29 days has no day 30.
How the lunar calendar is computed
The tool follows the astronomical method published by Hồ Ngọc Đức, using Jean Meeus's formulas for the new moon and the sun's position, at Vietnam's time zone, UTC+7:
- A lunar month begins on the day of the new moon. If the new moon falls at 23:15, that day is still day 1.
- Month 11 is the month that contains the winter solstice (Đông chí).
- Normally there are 12 new moons from one month 11 to the next. When there are 13, the year gets a leap month: the first month after month 11 in which the sun does not enter a new 30° sign (no "major term"). It takes the number of the month before it. In 2025 the month that began on 25 July had no major term, so it became a second month 6 (6 nhuận), lasting until 22 August.
Tết therefore always lands on the second new moon after the winter solstice, or the third when a leap month 11 or 12 comes in between, which keeps it between 21 January and 20 February. Can chi names come from fixed cycles: years and days run through a 60-name cycle (2024 Giáp Thìn, 2025 Ất Tỵ, 2026 Bính Ngọ), and month 1 is always a Dần month whose stem depends on the year's.
Vietnam is not China: when the dates differ
The Chinese calendar uses exactly the same rules at UTC+8. Most of the time the hour of difference changes nothing, but when a new moon falls between 23:00 and midnight in Vietnam it is already the next day in China. In 2007 the new moon was at about 23:15 on 17 February in Vietnam, so Tết Đinh Hợi was on 17 February in Vietnam and 18 February in China. In 1985 the effect was a whole month: the winter solstice of 1984 fell just before midnight on 21 December in Vietnam but after midnight in China, month 11 started a month earlier in Vietnam, and Tết Ất Sửu came on 21 January 1985 instead of 20 February. Vietnam then inserted a leap month 2 (21 March to 19 April 1985) and the two calendars met again.
North Vietnam switched its official time and calendar from UTC+8 to UTC+7 in 1967, and the South followed after 1975. That is why Tết 1968 was on 29 January in the North and 30 January in the South. This tool applies UTC+7 to every year, so for years before 1967 (before 1975 in the South), and for the 19th century, when the court calendar was computed with other methods, an occasional date can differ by a day from what was printed at the time. For birthdays use the age calculator, and to count days between two solar dates the date calculator. The full story with worked examples is in how the Vietnamese lunar calendar works.
