TL;DR: Tet Nguyen Dan in Vietnam and Chinese New Year fall on the same date in most years, but some years differ by one day and rare years by a whole month. The cause lies in two places: the new moon (soc) is calculated in different time zones, and the leap month rule can produce two different results when a lunar month contains no trung khi. A one-day difference usually comes from the time zone, while a whole-month difference usually comes from the leap month being placed in a different position.
Why do Vietnam and China celebrate Tet Nguyen Dan on different days?
Tet Nguyen Dan, also called Tet ca or Tet ta, marks the start of the lunar year. In most years, Vietnam and China welcome Tet on the same solar date. But every few years one side differs by a day, and in rare cases by a whole month. This phenomenon has caused debate, and at times it was blamed on one country or the other calculating the calendar incorrectly. In fact the cause lies elsewhere: both sides use the same kind of calendar, but calculate it in two different time zones and apply leap month rules that can produce two different results.
Is the Vietnamese lunar calendar a separate calendar?
The lunisolar calendar takes one revolution of the Moon around the Earth as one month, and combines it with one revolution of the Earth around the Sun as one year, joining the two into a lunisolar calendar[1].
In principle, the Vietnamese and Chinese calendars share this same root. The difference lies in which meridian the calendar makers stand on and which time zone they take as standard.
How are months and years calculated in a lunisolar calendar?
To understand why the dates differ, three concepts must be grasped: soc, trung khi, and the leap month.
- Soc is the moment when the Moon, Earth, and Sun are aligned, with the Moon in the middle, that is, the new moon. The day containing the soc point is taken as the first day of the lunar month.
- Trung khi are the markers dividing the solar year into 12 parts: Dong chi, Dai han, Vu thuy, Xuan phan, Coc vu, Tieu man, Ha chi, Dai thu, Xu thu, Thu phan, Suong giang, Tieu tuyet. There are also 12 intercalary tiet khi between them, collectively called tiet khi.
- The leap month is an added month to compensate for the gap between the lunar year (about 354 days) and the solar year (about 365 days).
The common rule for placing the leap month in a lunisolar calendar is: whichever month of the year contains no trung khi is taken as the leap month of the preceding month. This is precisely where the two calendars most easily diverge.
Why does the date differ by one day?
A one-day difference almost always comes from calculating the soc point in two different time zones. Vietnam calculates by Vietnam time. When the soc point falls in the gap between the two zones, that is, between midnight and one in the morning Vietnam time, the same soc point is recorded on two different days on the two sides. Whichever side records it one day earlier has its first day of the month one day earlier, and Tet differs by exactly one day.
This is a close-call difference, not a calendar error. The same astronomical event, viewed from two different meridians, is recorded on different days. Time zone conversion directly affects the determination of the start of the Ty hour, that is, the day-change marker[2]. The same principle applies exactly to determining the soc day.
Why do some years differ by a whole month?
A whole-month difference is rare and the cause usually lies not in the time zone but in the leap month. A lunar year has 12 or 13 months. When there is a leap month, its position is determined by the rule "the month containing no trung khi." If a month without trung khi falls near a certain marker, the two sides may choose different months as the leap month, or place the leap month at different positions in the year. Then all subsequent months are pushed one month apart, and the next year's Tet differs by a whole month.
Another case also leads to a one-month difference: the two sides differ on whether to take the winter solstice as the marker for month 11, or which time zone's soc point to use for determining month 11. Once month 11 is placed differently, the entire sequence of following months shifts accordingly.
What does the table comparing the two types of difference show?
| Type of difference | Main cause | Frequency | How to recognize it |
|---|---|---|---|
| One-day difference | The soc point falls in the gap between UTC+7 and UTC+8 | Relatively frequent | Neither side has a leap month in a disputed position, only the first day of the month differs |
| Whole-month difference | The leap month is placed at different positions, or lunar month 11 is determined differently | Rare | There is a leap month in the year, and its position does not match between the two sides |
How did calendar debates unfold in Vietnamese history?
The Tet date difference is not only a technical calculation matter; it was once a matter of state. Old histories record many passages where the court took an interest in calendar making, appointed officials to oversee calendar affairs, and at times had to adjust the calculation method to match the tiet khi. The feudal state issuing the calendar was a way to assert control over time, and also a way to ensure that harvests and rituals did not fall out of step.
Phan Khoi, in a 1934 article discussing Tet Nguyen Dan, raised the issue from a different angle: Tet is just the first day of the year, and celebrating Tet is a psychological matter of the nation, not a matter of heaven's decree[3]. This framing shows that as early as the beginning of the 20th century, Vietnamese intellectuals regarded Tet as a cultural convention open to discussion, not an inviolable marker.
So is Vietnamese Tet "more correct" or "more wrong" than Chinese New Year?
The lunisolar calendar is an astronomical calendar, and astronomy depends on the observation point: the hour must be based on the place of birth, so a calendar calculated by the time of one place may differ from one calculated by the time of another place without either being wrong.
It is worth noting that the current Vietnamese calendar is calculated by Vietnam time, so in principle it is Vietnam's own calendar, not a copy of the Chinese calendar. That is precisely why there are years with a date difference, and those years are no one's fault.
How should you look up Tet for the purpose you need?
When looking up the Tet date, you should look it up in the Vietnamese calendar, since that is the calendar used for holidays, offering days, and days off in Vietnam. If you need to compare with a foreign calendar, remember that a date difference does not mean one side recorded it wrongly. For important matters such as marriage, ground-breaking, and death anniversaries, the reference point remains the Vietnamese lunar date. And if you only want to know the date of Tet this year, the answer lies in the Vietnamese calendar, with no need to compare with any other calendar.
Why is Vietnam's Tet different from China's Tet?
Vietnamese Tet differs from Chinese New Year for two reasons: the time zone used to determine the soc point, and the rule for placing the leap month. A one-day difference is a time zone matter; a whole-month difference is a leap month matter. Both are results of an astronomical calendar calculated from its own observation point, not a sign that either side calculated incorrectly.[1][2][3]
