Why Vietnam's lunar calendar sometimes differs from China's by one day
Many people wonder why Vietnam and China sometimes celebrate Tet Nguyen Dan one day apart, even though both use a lunar calendar. The answer lies in the calendar calculation rules and the legal meridian, not in regional customs.
The core reason: different legal meridians
According to calendar researcher Tran Tien Binh, Vietnam and China apply the same set of lunar calendar rules: the first day of the month is the new moon day, an ordinary year has 12 months, a leap year has 13 months, the winter solstice always falls in month 11, and in a leap year the month without a major solar term is the leap month. The only difference lies in the legal meridian used for calculation: Vietnam uses 105 degrees East (UTC+7), China uses 120 degrees East (UTC+8).
This rule was established by Decision No. 121-CP dated August 8, 1967, signed by the late Prime Minister Pham Van Dong, fixing Vietnam's official time zone as UTC+7 and requiring the lunar calendar used in Vietnam to be calculated according to this official time zone.
Why a few minutes can shift the date
A lunar month is measured by the distance between two consecutive new moon points, rounded to 29 days (short month) or 30 days (full month). The length of a lunar month ranges from 29.27 to 29.84 days, averaging 29.53 days. If the new moon point falls at any time between 0 and 24 hours in a day, that day is the first day of the lunar month.
Therefore, if the new moon point falls close to midnight, a difference of just a few dozen seconds or minutes can push it into another day. Binh cites examples: according to a 2002 publication by the Purple Mountain Observatory (China), the dates September 28, 2057, September 4, 2089, and August 7, 2097 were miscalculated to the following day, changing the length of the corresponding months, because the new moon points fell at 23:59, 23:57, and 23:59.
The example of Tet Mau Than 1968
In 1968, Vietnam began using a calendar calculated according to UTC+7. As a result, Vietnam celebrated Tet one day earlier than China. The reason: December 31, 1967 corresponded to the first day of the twelfth lunar month in both Vietnamese and Chinese calendars, but the next new moon point occurred at 23:29 Vietnam time on January 29, 1968, so the twelfth month in the Vietnamese calendar had only 29 days (short month). At that moment, Beijing time was already 0:29 on January 30 (full month). Thus, the first day of Tet Mau Than 1968 in Vietnam was one day earlier than in China.
Vietnam's lunar calendar is lunisolar
One important note: what Vietnamese people call "am lich" (lunar calendar) is actually a lunisolar calendar, following both the lunar cycle and the Earth's movement around the Sun. According to Tran Tien Binh, Vietnam's lunar calendar should accurately be called a lunisolar calendar. Each solar term corresponds to a specific position of the Earth around the Sun, for example Xuan phan at 0 degrees, Thanh minh at 15 degrees, Coc vu at 30 degrees, Kinh trap at 345 degrees.
Because of the roughly 11-day gap between the lunar year and the solar year, a leap month appears every few years. The year At Ty 2025 has a leap month 6, totaling 384 days, from January 29, 2025 to February 16, 2026.
Tet customs across the North, Central, and South
The reference sources collected for this article only address the astronomical and calendar-calculation aspects, not the specific Tet customs of each region. Therefore, detailed comparison of customs across the three regions is incomplete in the available sources. Below are the shared points recorded, with clear notes on what is missing.
The North
The available sources do not provide a specific description of Tet customs in the North. There is only general information: Tet Nguyen Dan marks the transition to a new year, carrying the meaning of reunion and an auspicious beginning, tied to Eastern culture and traditional festivals. Information on rituals and typical offerings of the North is not fully covered in the sources.
The Central region
Similarly, the reference sources do not mention Tet customs specific to the Central region. The articles focus on how the lunar calendar is calculated, the leap month phenomenon, the 30th day of Tet, and the date difference between Vietnam and China. Information on offerings, timing, and typical rituals of the Central region is not available in the sources.
The South
The reference sources also do not mention Tet customs specific to the South. Therefore, the comparison of customs across the three regions in this article only records the information gap, without drawing conclusions about specific differences or similarities.
Summary comparison table
| Criterion | The North | The Central | The South |
|---|---|---|---|
| Main offerings | No information in sources | No information in sources | No information in sources |
| Tet timing | Same Tet Nguyen Dan date according to Vietnam's lunisolar calendar | Same Tet Nguyen Dan date according to Vietnam's lunisolar calendar | Same Tet Nguyen Dan date according to Vietnam's lunisolar calendar |
| Typical rituals | No information in sources | No information in sources | No information in sources |
| Calendar basis | UTC+7, meridian 105 degrees East | UTC+7, meridian 105 degrees East | UTC+7, meridian 105 degrees East |
The table shows the only shared point confirmed by the sources: all three regions use the same lunisolar calendar calculated according to Vietnam's UTC+7. Differences in offerings, timing, and rituals specific to each region require additional in-depth cultural sources for a full comparison.
Conclusion
The Tet date difference between Vietnam and China is an astronomical phenomenon, stemming from the two countries using different legal meridians. Within the available reference sources, a detailed comparison of Tet customs across the North, Central, and South cannot yet be made. Readers should consult additional regional cultural materials to explore this aspect further.
