Why do two calendars give different lunar dates for the same solar day?
The same solar day can carry two different lunar dates on two calendars because they are calculated in different legal time zones, or because one of them uses data from an unofficial source. In the typical case of Vietnamese and Chinese calendars, the main cause is that Vietnam calculates by UTC+7 while China calculates by UTC+8. When the new moon moment falls near midnight, that one hour gap is enough to push the first day of the month to the next day in one country while it remains within the old day in the other.
This is not a rare event. Nhan Dan newspaper has reported that the sixth lunar month of 2006 differed by one day between Vietnam and China, and from the first day of the seventh lunar month the two countries matched again until the end of the year. By the same mechanism, Vietnam's Lunar New Year has in some years arrived one day before China's.
How is the first day of a lunar month determined?
The first day of a lunar month is the day that contains the new moon moment, when the Earth, the Moon and the Sun line up in that order and the Moon turns its dark side toward the Earth. This is the moonless day, known in folk language as the new moon day. According to Mr. Tran Tien Binh, a former staff member of the Calendar Research Department at the Vietnam Academy of Science and Technology, if the new moon moment falls anywhere between 0 and 24 hours of a given day, 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. That average equals 29 days 12 hours 44 minutes 2.8 seconds, and because the fractional part never comes out even, 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 motion of three bodies, the Earth, the Moon and the Sun, not merely on the Moon's waxing and waning cycle as many people assume. 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 shapes of the Earth and the Moon. As a result, an error of just a few dozen seconds can cause a month to be classified as short instead of full, or the reverse.
Mr. Tran Tien Binh cites a 2002 publication by the Purple Mountain Observatory in China: the dates 28 September 2057, 4 September 2089 and 7 August 2097 were miscalculated to the following day, changing the lengths of the corresponding months. The new moon of 28 September 2057 fell at 23:59, the new moon of 4 September 2089 fell at 23:57, and the new moon of 7 August 2097 fell at 23:59. Only minutes before midnight, yet enough to reverse the result.
Where do legal time zones come in?
Legal time zones come in because they determine which day contains the new moon moment, and therefore which day is the first day of the lunar month. The new moon moment is calculated in international standard time, but when converted to local time for printing the calendar, each country adds the offset of the time zone it has chosen. If the result still falls within the day, the first day falls on that day; if it crosses midnight, the first day falls on the next day.
Vietnam has chosen UTC+7 as its official time, with a central meridian of 105 degrees east. China has chosen UTC+8, with a central meridian of 120 degrees east. The two zones differ by exactly one hour. Nhan Dan newspaper cites Decision No. 134/2002/QD-TTg dated 14 October 2002, whose Article 1 states that the seventh time zone under the international time zone system is the official time of Vietnam. Earlier, Decision No. 121-CP dated 8 August 1967 had also stipulated that the official time zone of the country is the seventh zone and that the lunar calendar used in Vietnam is calculated by this official time zone.
A concrete example: the sixth lunar month of 2006
In 2006, the new moon moment for the sixth lunar month in international standard time was 16:06:21. Adding 7 hours for Vietnam time gives 23:06:21, still within the day, so the Vietnamese calendar records the first day of the sixth lunar month as 25 June in the solar calendar. Adding 8 hours for Beijing time gives 00:06:21 the next day, so the Chinese calendar records the first day of the sixth lunar month as 26 June. The solar date and the sexagenary day remain the same between the two countries; only the lunar date differs by one day.
A concrete example: Lunar New Year 2007
The new moon moment for the first day of Tet 2007 was 16:15:23 in international time. Adding 7 hours gives 23:15:23, still within the day, so the Vietnamese calendar records the first day of Tet as 17 February. Adding 8 hours crosses midnight into the next day, so the Chinese calendar records the first day of Tet as 18 February. Again the solar date and the sexagenary day are the same; only the lunar date differs.
Years when Vietnamese and Chinese Tet fell on different dates
According to Nhan Dan newspaper, at the end of the 20th century there were three occasions when Vietnamese Tet differed from Chinese Tet by one day or one month. In the years Mau Than 1968 and Ky Dau 1969, Vietnamese Tet came one day earlier. In the year At Suu 1985, Vietnamese Tet came one month earlier. In the 21st century, according to the forecast cited in the article, there are three Lunar New Year dates in Vietnam that come one day before Chinese Tet, falling in 2007, 2030 and 2053.
Mr. Tran Tien Binh explains the 1968 case in detail. 31 December 1967 corresponded to the first day of the twelfth month in both the Vietnamese and Chinese calendars, but the next new moon moment occurred at 23:29 Vietnam time on 29 January 1968, so the twelfth month in the Vietnamese calendar had only 29 days, a short month. In Beijing time it was already 00:29 on 30 January, so the twelfth month in the Chinese calendar was a full month. As a result, the first day of Tet, Mau Than 1968, in Vietnam came one day earlier than in China.
Why do calendars sold on the market also differ?
Calendars sold on the market can also differ because not every one of them uses data from an official source. Nhan Dan newspaper quotes a representative of the State Calendar Board as saying that the Board is tasked with researching and managing the state calendar and provides the official calendar tables each year to all calendar publishers nationwide. Taking information from a source other than the State Calendar Board does not match the calendar of our country and is only for reference. In cultural exchange, many kinds of research and reference calendars circulate on the market, so users may come across copies that record lunar dates different from the official calendar.
According to information in Nhan Dan newspaper, to ensure accuracy, publishers must comply with Circular No. 61/2006/TT-BVHTT dated 15 June 2006 of the Ministry of Culture and Information, which requires that calendar printing use official data from the State Calendar Board.
Why do Vietnamese and Chinese calendars sometimes have opposite full and short months?
Vietnamese and Chinese calendars sometimes have opposite full and short months for the same time zone reason, except that the consequence falls on the length of the month rather than on the first day. Nhan Dan newspaper has recorded the phenomenon of one month being short and the next being full in the Vietnamese calendar, while the same month is full and the next is short in the Chinese calendar, after which the two calendars match again. Mr. Tran Tien Binh also says that statistics on whether the twelfth month is full or short across years do not carry much meaning, because everything depends on the accurate calculation at each specific moment.
| Year | Twelfth month in the Vietnamese calendar | Note |
|---|---|---|
| 2014 to 2021 | Full, 30 days (except 2022) | According to statistics by Mr. Tran Tien Binh |
| 2022 | Short, 29 days | The twelfth month had no 30th day |
| 2023, 2024 | Full, 30 days | 2024 had a 30th day of Tet, Giap Thin |
| 2025 to 2032 | Short, 29 days | No 30th day of the twelfth month |
| 2033 | Full, 30 days | First return of a 30th day of the twelfth month |
The Vietnamese calendar is actually a lunisolar calendar
It should be made clear that what Vietnamese people call the lunar calendar is actually a lunisolar calendar, combining both the Moon's cycle and the hot and cold weather cycle caused by the Earth's orbit around the Sun. According to VietNamNet, a purely lunar year has only 354 or 355 days, while the hot and cold weather cycle lasts about 364 days, leaving 10 to 11 extra days each year, more than a month over three consecutive years. To match the weather cycle, people in the past added one month in the third year, called an intercalary month; that year had 13 months and lasted 384 or 385 days.
Vietnamese and Chinese lunar calendar calculations both apply the same set of rules: the first day of the month is the new moon day; an ordinary year has 12 months and an intercalary year has 13 months; the winter solstice always falls in the eleventh month; in an intercalary year, a month without a major solar term is the intercalary month and is numbered together with the month before it, with the word intercalary added. If an intercalary year has two months without a major solar term, the first month after the winter solstice is treated as the intercalary month. The only difference between the two countries lies in the legal meridian used for calculation: Vietnam uses 105 degrees east, China uses 120 degrees east.
On the origin of the lunar calendar
According to VietNamNet, citing the Historical Research page and Professor Tran Ngoc Them's book The Basis of Vietnamese Culture, a calendar based on the Moon's motion did not begin in China as many people assume. 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 Moon's motion. The Roman Empire before Julius Caesar, around 40 BC, also used this kind of lunar calendar. Professor Tran Ngoc Them says the lunar calendar originated in the Mesopotamian cultural region, based on the Moon's motion with a cycle of 29.53 days and 354 days per year.
So when two calendars differ, which one should be followed?
When two calendars differ, users in the country should follow the Vietnamese state calendar, that is, the calendar provided by the State Calendar Board and calculated by UTC+7. Nhan Dan newspaper states that lunar dates printed on calendars that do not follow information provided by the State Calendar Board are wrong, while the Vietnamese state calendar is correct for our country. This does not mean the Chinese calendar is wrong; rather, the Chinese calendar is correct for the Chinese lunar calendar, because each country calculates by its own legal time.
It should also be noted that the solar date and the sexagenary day between the two countries remain the same in all the cases mentioned. Only the lunar date, the number of days in the lunar month and the Lunar New Year date can differ. Understanding this mechanism correctly helps avoid confusion when looking up death anniversaries, holidays, or when comparing perpetual calendars from different sources. In practice, researchers still do not fully agree on some technical points in calendar calculation for times far in the past and future, but for the current calendar, the legal time zone convention is clearly the most decisive factor.
