Chinese and Vietnamese calendar differences
Shared structure
The Chinese and Vietnamese calendars belong to the same broad lunisolar tradition. Their months follow the Moon, while their month numbering and leap months keep the year connected to the Sun and the seasons. Twelve average lunar months total about 354 days, roughly eleven days short of a seasonal year. Adding an occasional whole month prevents that shortfall from carrying New Year steadily through the seasons.
In both modern systems, lunar day 1 is the civil day containing the astronomical new moon, when the Moon and Sun have the same longitude along the ecliptic as seen from Earth's centre. The month does not begin with the first visible crescent. A month contains 29 or 30 civil days. The winter solstice belongs to month 11, providing an annual reference point for numbering the months.
Principal terms are the twelve alternate members of the 24 solar terms, at 30-degree intervals of the Sun's ecliptic longitude. They help determine where a leap month belongs. Count the months from the beginning of one winter-solstice month up to, but not including, the next winter-solstice month. When there are thirteen, the first month in that interval without a principal term is the leap month. It repeats the preceding month's number. When there are twelve, no leap month is inserted in that interval.
A leap month therefore cannot be identified simply by finding any month without a principal term. Its place within the solstice-to-solstice sequence matters. “Leap month 6” means an additional lunar month following ordinary month 6.
The time-zone difference
Modern Vietnamese calculation uses UTC+7, corresponding to the standard meridian 105° east. Modern Chinese calculation uses UTC+8, corresponding to 120° east. These reference times determine the civil dates assigned to new moons and principal terms. They remain the same when the calendar is used abroad.
An event near midnight can fall on different dates in the two systems. If it is a new moon, the lunar month can begin a day earlier in Vietnam. A principal term falling on different sides of a month boundary can also change which month contains it, affecting month numbering and leap-month placement.
Month numbering around New Year 1985
| Event or month start | Vietnamese calendar | Chinese calendar |
|---|---|---|
| December solstice, 1984 | December 21 | December 22 |
| Month beginning November 23, 1984 | Month 11 | Leap month 10 |
| Month beginning December 22, 1984 | Month 12 | Month 11 |
| Month beginning January 21, 1985 | Month 1: Tet | Month 12 |
| Month beginning February 20, 1985 | Month 2 | Month 1: Chinese New Year |
Here the solstice fell on different local dates, while the December new moon fell on December 22 in both systems. In Vietnam, the solstice belonged to the month that had begun in November. In the Chinese calendar, it belonged to the new month beginning on December 22. Since each calendar assigns the solstice to month 11, the following months received different numbers. Tet occurred on January 21, 1985, and Chinese New Year on February 20, thirty days later.
Most new moons and principal terms fall on the same civil date in both calendars. Differences arise around particular boundaries, so neither calendar stays permanently a day or a month ahead.
Historical conversion
Vietnam's calendar history includes periods of shared Chinese calendar use, separate Vietnamese calculation and different northern and southern practice. The present UTC+7 basis was adopted for calendar calculation in northern Vietnam in 1968 and nationwide in 1976. Changes to civil clocks did not always coincide with changes to the calendar's reference time.
A historical date needs its place, original year designation, lunar month and day, and any leap-month mark. Conversion should use the calendar in force there at the time. Some medieval Vietnamese calendars cannot be fully reconstructed from surviving material. Where a conversion substitutes a Chinese calendar or applies a later rule, that approximation belongs in the record alongside the result.