TL;DR: Two calendars showing different lunar dates for the same solar day usually happens because they are calculated for different legal time zones, typically Vietnam using UTC+7 (105°E) and China using UTC+8 (120°E). When the new moon moment falls near midnight, a one hour difference can push the first day of the lunar month to the previous or next day, shifting the whole month and the Tết Nguyên đán date. Also, calendars printed in the market may come from unofficial sources and therefore differ. Tết date differences between Vietnam and China are not related to regional customs, because all of Vietnam uses one calendar based on UTC+7.
Why do two calendars show two different lunar dates for the same solar date?
For the same solar date, two calendars can show two different lunar dates because they are calculated according to two different legal time zones, or because one of the two takes its data from an unofficial source. In the typical case between the Vietnamese calendar and the Chinese calendar, the main cause is that Vietnam calculates by the seventh time zone while China calculates by the eighth time zone. When the new moon point (the moment of no moon) falls near midnight, that roughly one hour difference is enough to push the date of the first day of the lunar month to the following day in one country while it still falls on the old day in the other.
This is not a rare phenomenon. Nhân Dân newspaper once reported that the sixth lunar month of 2006 in Vietnam and China differed by one day, and that by the first day of the seventh lunar month the two countries coincided again until the end of the year. By the same mechanism, Vietnam's Tết Nguyên đán (Lunar New Year) falls one day before China's Lunar New Year in some years.
How is the first day of the lunar month determined?
The first day of the lunar month is the day that contains the new moon point, that is, the moment when the Earth, the Moon and the Sun lie in a straight line in that order, when the Moon turns its dark half toward the Earth. This is the day of no moon, popularly also called the new moon day. According to Mr Trần Tiến Bình, a former staff member of the Calendar Research Office of the Vietnam Academy of Science and Technology, if the new moon point is found to fall at any moment between 0 and 24 hours of a day, then that day is taken as the first day of the lunar month.
The length of a lunar month is the interval between two consecutive new moon days, rounded to 29 days (short month) or 30 days (full month). The actual interval ranges from about 29.27 to 29.84 days, averaging 29.53 days. This average figure is equivalent to 29 days 12 hours 44 minutes 2.8 seconds, and precisely because the fractional part is never round, a lunar month cannot be fixed at 29 or 30 days.
Why is a lunar month not fixed at 29 or 30 days?
A lunar month depends on the simultaneous activity of three celestial bodies, the Earth, the Moon and the Sun, and not only on the waxing and waning cycle of the Moon as many people mistakenly believe. Calculating the motion of celestial bodies is very complex, especially for the Moon, because it is disturbed by the gravitational pull of the Sun, the Earth and other planets, plus the irregular shape of the spheres of the Earth and the Moon. Therefore, an error of just a few tens of seconds can lead to a month being classified as short instead of full, or the reverse.
Mr Trần Tiến Bình cites a publication issued in 2002 by the Purple Mountain Observatory (China): the dates 28.9.2057, 4.9.2089 and 7.8.2097 were miscalculated to the following day, changing the lengths of the corresponding months. The new moon point of 28.9.2057 fell at 23 hours 59, the new moon point of 4.9.2089 fell at 23 hours 57, and the new moon point of 7.8.2097 fell at 23 hours 59. Only a few minutes before midnight, but enough to reverse the result.
Where does the legal time zone come in?
The legal time zone comes in at the point where it determines which day contains the new moon point, and therefore determines which day is the first day of the lunar month. The new moon point is calculated in international standard time, but when converted to local time to record the calendar, each country adds the hour difference of the time zone it has chosen. If the result still falls within the day, then the first day falls on that day; if it passes midnight, then the first day falls on the following day.
Vietnam chose the seventh time zone as its official time, with a central meridian of 105 degrees east longitude, that is GMT+7. China chose the eighth time zone, with a central meridian of 120 degrees east longitude, that is GMT+8. These two zones differ by exactly one hour. Nhân Dân newspaper cites Decision No. 134/2002/QĐ-TTg dated 14-10-2002, in which Article one states clearly that the seventh time zone under the international time zone system is taken as the official time of Vietnam. Earlier, Decision No. 121-CP dated 8.8.1967 had also stipulated that the official time zone of our country is the seventh time zone and that the lunar calendar used in Vietnam is calculated according to this official time zone.
A specific example: the sixth lunar month of 2006
In 2006, the new moon point of the sixth lunar month in international standard time was 16 hours 6 minutes 21 seconds. Adding 7 hours for Vietnam time, we get 23 hours 6 minutes 21 seconds, still within the day, so the Vietnamese calendar records the first day of the sixth lunar month as corresponding to 25-6 in the solar calendar. Adding 8 hours for Beijing time, we get 0 hours 6 minutes 21 seconds on the following day, so the Chinese calendar records the first day of the sixth lunar month as corresponding to 26-6 in the solar calendar. The solar date and the can chi day between the two countries remain the same, only the lunar date differs by one day.
A specific example: Tết Nguyên đán 2007
The new moon point calculated for the first day of Tết in 2007 was 16 hours 15 minutes 23 seconds international time. Adding 7 hours gives 23 hours 15 minutes 23 seconds, still within the day, so the Vietnamese calendar records the first day of Tết as corresponding to 17-2 in the solar calendar. Adding 8 hours passes midnight into the following day, so the Chinese calendar records the first day of Tết as corresponding to 18-2 in the solar calendar. It is still the same solar date and the same can chi day, only the lunar date differs.
The times when Vietnam's Tết and China's Tết have differed
According to Nhân Dân newspaper, at the end of the 20th century there were three occasions when Vietnam's Tết differed from China's Tết by one day or one month. In the year Mậu Thân 1968 and the year Kỷ Dậu 1969, Vietnam's Tết came one day earlier. In the year Ất Sửu 1985, Vietnam's Tết came one month earlier. In the 21st century, according to the forecast given in the article, there are three Lunar New Year dates when Vietnam's Tết Nguyên đán comes one day before China's Tết, falling in the years 2007, 2030 and 2053.
The case of 1968 is explained in detail by Mr Trần Tiến Bình. The date 31.12.1967 corresponded to the first day of the twelfth lunar month in both the Vietnamese and the Chinese calendars, but the next new moon point occurred at 23 hours 29 minutes Vietnam time on 29.1.1968, so the twelfth lunar month in the Vietnamese calendar had only 29 days, that is a short month. In Beijing time it was already 0 hours 29 minutes on 30.1, so the twelfth lunar month in the Chinese calendar was a full month. The result is that the first day of Tết Nguyên đán Mậu Thân 1968 in Vietnam was one day earlier than in China.
Why do calendars sold on the market also record different dates?
Calendars sold on the market can also record different dates because not every copy takes its data from an official source. Nhân Dân newspaper cites a representative of the State Calendar Committee as saying that the State Calendar Committee has the task of researching and managing the State calendar, and annually provides the official calendar table to all calendar publishers nationwide. If information is taken from a source other than the State Calendar Committee, it does not match our country's calendar and serves only for reference. In cultural exchange, there are many kinds of research and reference calendars on the market, so users may come across copies that record a lunar date different from the official calendar.
According to information in Nhân Dân newspaper, to ensure accuracy, publishers need to implement Circular No. 61/2006/TT-BVHTT dated 15-6-2006 of the Ministry of Culture and Information, which stipulates that calendar printing must take official data from the State Calendar Committee.
Why do the Vietnamese and Chinese calendars sometimes have opposite full and short months?
The Vietnamese and Chinese calendars sometimes have opposite full and short months for the same reason of time zones, except that the consequence falls on the length of the month rather than on the first day of the month. Nhân Dân newspaper once noted the phenomenon of this month being short and the next month full in the Vietnamese calendar, while this month was full and the next month short in the Chinese calendar, and afterwards the two calendars coincided again. Mr Trần Tiến Bình also said that statistics on whether the twelfth lunar month is full or short across the years do not carry much meaning, because everything depends on the results of precise calculation at each specific moment.
| Year | Twelfth lunar month in the Vietnamese calendar | Note |
|---|---|---|
| 2014 to 2021 | Full 30 days (except 2022) | According to statistics by Mr Trần Tiến Bình |
| 2022 | Short 29 days | The twelfth lunar month has no 30th day |
| 2023, 2024 | Full 30 days | 2024 has a 30th day of Tết Giáp Thìn |
| 2025 to 2032 | Short 29 days | No 30th day of the twelfth lunar month |
| 2033 | Full 30 days | The first time a 30th day of the twelfth lunar month returns |
The Vietnamese calendar is in fact a lunisolar calendar
What needs to be made clear is that the calendar that Vietnamese people still call the lunar calendar is in fact a lunisolar calendar, that is, it combines both the Moon's cycle and the cycle of hot and cold weather caused by the Earth's revolution around the Sun. According to VietNamNet newspaper, a purely lunar year has only 354 or 355 days, while the weather cycle of changing hot and cold is about 364 days, so each year has 10 to 11 extra days, and three consecutive years accumulate more than one month. To fit the hot and cold cycle, the ancients added one month to the third year, called the leap month; that year has 13 months and is 384 or 385 days long. For a recent example, the year Ất Tỵ 2025 has a leap sixth month, that is two sixth months, 384 days in total, counted from 29.1.2025 to 16.2.2026, leading to Tết Bính Ngọ on 17.2.2026.
The method of calculating the lunar calendar in Vietnam and China both apply one common set of 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 the eleventh month; in a leap year, a month without a major solar term is the leap month and is numbered together with the month before it, with the word leap added. If in a leap year there are two months without a major solar term, then the first month after the winter solstice is regarded as the leap month. The only difference between the two countries lies in the legal meridian used for calculation: Vietnam uses 105 degrees east longitude, China uses 120 degrees east longitude.
To explain further the term major solar term just used: each solar term corresponds to a particular position of the Earth on the ecliptic, calculated by solar longitude. The spring equinox corresponds to 0 degrees, Thanh minh corresponds to 15 degrees, Cốc vũ corresponds to 30 degrees, Kinh trập corresponds to 345 degrees. Solar terms whose longitude is a multiple of 30 degrees are called major solar terms, while the remaining solar terms are called solar terms. A normal lunar month contains one major solar term; any month that does not contain a major solar term is the basis for marking the leap month.
On the origin of the lunar calendar
According to VietNamNet newspaper citing the page Nghiên cứu lịch sử and the book Cơ sở văn hóa Việt Nam by Professor Trần Ngọc Thêm, a calendar based on the motion of the Moon did not begin in China as many people mistakenly believe. The Sumerian civilization that developed beside the Tigris river more than 6000 years ago, in the land that later became Baghdad, the capital of Iraq, already knew how to make a calendar based on the motion of the Moon. The Roman Empire before Julius Cesar, around the year 40 before the Common Era, also used this kind of lunar calendar. Professor Trần Ngọc Thêm says the lunar calendar arose from the Mesopotamian cultural region, based on the motion of the Moon with a cycle of 29.53 days, with 354 days in each year.
So when two calendars record different dates, which one should be followed?
When two calendars record different dates, users in the country should follow the Vietnamese State calendar, that is, the calendar provided by the State Calendar Committee and calculated according to the seventh time zone. Nhân Dân newspaper affirms that the lunar dates printed on kinds of calendars that do not follow the information provided by the State Calendar Committee are wrong, while the Vietnamese State calendar is correct for our country. This does not mean the Chinese calendar is wrong, but that the Chinese calendar is correct for the Chinese lunar calendar, because each country calculates according to its own legal time.
It should also be noted that the solar date and the can chi day between the two countries remain the same in every case mentioned. Only the lunar date, the number of days in the lunar month and the date of Tết Nguyên đán can differ. Understanding this mechanism correctly helps avoid confusion when looking up death anniversary dates, holidays, or when comparing perpetual calendars from different sources. In practice, researchers still do not fully agree on some technical points in calendar calculation at times far in the past and future, but with the current calendar, the convention of the legal time zone is the clearest deciding factor.
Is the difference in Tết dates between Vietnam and China related to regional customs?
No. The difference in Tết dates between Vietnam and China is only due to the different legal meridians used for calendar calculation, Vietnam taking 105 degrees east longitude while China takes 120 degrees east longitude, and has nothing to do with regional customs. Within the country, all three regions, North, Central and South, use one shared calendar calculated according to the seventh time zone, so there is no question of one region celebrating Tết on a different date from another because of custom.
Two different matters need to be distinguished. One is the date of Tết Nguyên đán, which is decided by astronomy and the legal meridian, so the whole country has one unified date. Two is the way Tết is celebrated, that is, offerings, rituals and the timing of worship, which can differ between regions, but that is a matter of custom and does not change the date of Tết. The reference sources gathered for this article only mention the astronomical aspect and calendar calculation rules, and do not provide detailed descriptions of the separate Tết celebration customs of each region.
