TL;DR: Tử Vi counts the birth year by the lunar calendar, starting from the first day of the first lunar month, so a birth before Tết still belongs to the old year. Bát Tự and Hà Lạc instead use the Lập Xuân solar term as the marker, so a person born in the twelfth lunar month after Lập Xuân has already entered the new year's stem-branch, while a person born in the first lunar month before Lập Xuân is still counted in the old year. The two systems use two different markers, so the same birth date can yield two different stem-branch years.
Born before or after Tết: which age counts?
The short answer: it depends on the system you use. Tử Vi takes the first day of the first lunar month as its marker, while Bát Tự and the Hà Lạc method take the Lập Xuân solar term. A person born before Tết by the lunar calendar still carries the old year's age in Tử Vi, but if the birth date has already passed Lập Xuân, Bát Tự has recorded them in the new stem-branch year. This is not a contradiction, but two calendar conventions serving two different purposes.
Why do Tết and Lập Xuân rarely coincide?
Tết Nguyên Đán is the new-moon day that opens the first lunar month; between 1900 and 2100 it falls between 21 January and 20 February. Lập Xuân is the solar term when the Sun reaches 315 degrees of celestial longitude; in Vietnam time it falls on 4 February in 128 years, on 5 February in 30 years (1902 to 1972) and on 3 February in 43 years (2009 to 2099). One marker follows the Moon and the other the Sun, so the two days coincide in only five years (1905, 1924, 1992, 2038, 2087); in other years they are up to 15 days apart, and Lập Xuân can fall on any day from the 15th of the twelfth lunar month to the 15th of the first lunar month.
| Year | First day of Tết | Lập Xuân (Vietnam time) | Lập Xuân in the lunar calendar |
|---|---|---|---|
| 2025 | 29 January 2025 | 3 February 2025, 21:07 | 6th day of the first month |
| 2026 | 17 February 2026 | 4 February 2026, 02:55 | 17th day of the twelfth month |
| 2027 | 6 February 2027 | 4 February 2027, 08:42 | 28th day of the twelfth month |
| 2028 | 26 January 2028 | 4 February 2028, 14:30 | 10th day of the first month |
| 2029 | 13 February 2029 | 3 February 2029, 20:19 | 20th day of the twelfth month |
| 2030 | 2 February 2030 | 4 February 2030, 02:08 | 3rd day of the first month |
Vietnamese books on the Hà Lạc method make the same distinction. Hoàng Tuấn, in Kinh dịch và hệ nhị phân, writes that unlike other methods, which start the year on the first day of the first lunar month, the Hà Lạc method starts it at Lập Xuân, so in a year when Lập Xuân falls in the twelfth month, a person born after that point already counts in the new year.
Which marker does Tử Vi use for the birth year?
Tử Vi takes the birth date, month, and hour by the lunar calendar, and the birth year also by the lunar year. The book Tử Vi khảo luận by Hoàng Thường and Hàm Chương states: "Birth year, month, day, and hour: calculated by the lunar calendar. If you do not know how to calculate the lunar date and hour, rely on century calendars or on star-placement software that converts from the solar to the lunar calendar." Thus, for Tử Vi, a person born on the 28th of the twelfth lunar month still belongs to the old year until the passing of the New Year's Eve moment, and only on the first day of Tết do they enter the new year. The year-change marker is the New Year's Eve moment, not Lập Xuân.
How do Bát Tự and Hà Lạc use the Lập Xuân marker?
Bát Tự determines the Year pillar from the Lập Xuân solar term. A person born before Lập Xuân belongs to the Year pillar of the previous year; a person born after Lập Xuân belongs to the Year pillar of that year. Hà Lạc books call a birth in the twelfth lunar month after Lập Xuân 'lệch năm sinh tiến bộ' or 'lệch tiến' (forward birth-year shift), and a birth in the first lunar month before Lập Xuân 'lệch năm sinh thoái bộ' or 'lệch thoái' (backward birth-year shift).
For example, in 2024 Lập Xuân fell at 15:20 on 4 February, the 25th day of the twelfth month of the Quý Mão year, while Tết of Giáp Thìn was on 10 February. A person born at noon on 6 February 2024 (the 27th of the twelfth month) is Quý Mão in Tử Vi, but the Bát Tự Year pillar is already Giáp Thìn. Conversely, in 2023 Tết of Quý Mão was on 22 January and Lập Xuân at 09:33 on 4 February (the 14th of the first month); a person born on 1 February 2023 (the 11th of the first month) is Quý Mão in Tử Vi, but the Bát Tự Year pillar is still Nhâm Dần.
Table comparing the two systems
| Criterion | Tử Vi | Bát Tự, Hà Lạc |
|---|---|---|
| Year-change marker | First day of the first lunar month (New Year's Eve) | Lập Xuân solar term (3, 4 or 5 February) |
| Person born in the twelfth month after Lập Xuân | Still old year | Already new year (lệch tiến) |
| Person born in the first month before Lập Xuân | Already new year | Still old year (lệch thoái) |
| Calendar basis | Pure lunar calendar | Solar terms, 24 solar terms |
| Impact of wrong determination | Wrong Cục, wrong Thái Tuế and Lộc Tồn rings | Wrong Year pillar, wrong nạp âm, wrong đại vận |
Why does Bát Tự choose Lập Xuân instead of Tết?
Tam mệnh thông hội by Vạn Dân Anh (Ming dynasty) assigns two solar terms to each Bát Tự month, starting with "正月寅【立春雨水】" (the first month is the Dần month, covering Lập Xuân and Vũ Thủy). The book also notes: "节气有立春在十二月末却在十二月生得明年春气", meaning that in some years Lập Xuân falls at the end of the twelfth month, and a person born in the twelfth month after it has already received the spring qi of the next year. In the view of mệnh học (the study of destiny), the Bát Tự year and month are cycles of qi that follow the Sun, so the year begins at Lập Xuân rather than at Tết of the civil calendar.
Which cases are prone to age miscalculation?
Two groups are most easily confused:
- Born between 21 January and 20 February, roughly from the 15th of the twelfth lunar month to the 15th of the first lunar month: both Tết and Lập Xuân fall in this window, so the Tử Vi year and the Bát Tự year may differ. Check both the Tết date and the Lập Xuân date and time of the birth year.
- Born on the day of Lập Xuân: the birth hour is needed to know whether the birth came before or after the moment the term begins.
Are Tử Vi and Bát Tự contradictory?
Not exactly. The two systems serve two different purposes. Tử Vi places stars according to the lunar birth date and hour, so it needs a clear civil marker, which is New Year's Eve. Bát Tự describes the five-element interactions of stem and branch, so it needs the qi-term marker, which is Lập Xuân. A person can have an old-year Tử Vi year but a new-year Bát Tự year, and that does not make either chart wrong, as long as each system is set up according to its own convention.
However, one point needs attention: when converting a solar birth date to the lunar calendar for Tử Vi, the chart setter must use the Vietnamese lunar calendar, not the Chinese one, because the Vietnamese calendar is computed for UTC+7 and the Chinese for UTC+8, so in some years the two differ by a day and in some by a whole month (in 1985, Tết fell on 21 January in Vietnam and on 20 February in China). Hoàng Xuân Hãn, in his study Lịch và lịch Việt Nam (included in the collection La Sơn Yên Hồ Hoàng Xuân Hãn), examined the occasions on which the Vietnamese calendar diverged from the Chinese one.
Impact of determining the birth year incorrectly
With Tử Vi, a wrong birth year means a wrong year stem and branch, which can change the Cục, the Thái Tuế ring, the Lộc Tồn ring and the Tứ Hóa, so the whole chart shifts; the position of the Mệnh palace itself does not change, because it is placed from the birth month and hour. With Bát Tự, a wrong Year pillar leads to a wrong year nạp âm, a wrong stem for the Month pillar, wrong stem-branch relations with the other pillars, and possibly the wrong direction of the đại vận.
Nhật Nguyệt's suggestion
When looking up the age for a person born near Tết, two steps should be taken: first, look up the Lập Xuân date of the birth year to know which stem-branch year the person belongs to according to Bát Tự; second, look up the first day of Tết to know the lunar year used for Tử Vi. Record both results in the file, do not merge them into one. For a person born exactly on Lập Xuân, the birth hour is needed to determine before or after the term. This is a calendar lookup, not a fortune prediction, so it can be done accurately with a printed calendar or an independent spreadsheet.
