How solar-lunar date conversion works
This tool converts dates using the same astronomical method published by Ho Ngoc Duc for Vietnam's official lunar calendar: it locates new moons using orbital mechanics formulas and determines each lunar month by the sun's position, rather than using a fixed offset table. The calendar is anchored to UTC+7 (Vietnam time), which is the standard basis for Vietnamese am lich dates.
Because lunar months are defined by actual new moons and the leap-month rule depends on where the winter solstice falls, this calculation differs slightly from a simple fixed-cycle approximation and matches real published Vietnamese lunar calendars for 1900 through 2100.
How do I convert a date between the solar and lunar calendar?
This tool uses an astronomical algorithm that locates real new moons and tracks the sun's position to build the lunar calendar month by month, the same method Vietnam uses for its official am lich calendar. Example: the solar date February 10, 2024 converts to lunar date 1/1/2024 (the first day of the lunar new year, Tet), in the Year of the Dragon.
Steps to convert a date
- Choose Solar to Lunar or Lunar to Solar with the toggle at the top of the tool.
- For Solar to Lunar, pick any date between 1900 and 2100 using the date picker.
- For Lunar to Solar, enter the lunar day, month, and year, and check the leap-month box only if you specifically mean the intercalary (leap) version of that month.
- Read the converted date, plus the zodiac animal year shown alongside it.
- If Lunar to Solar shows an error, the day/month/leap combination you entered does not exist in that lunar year — double check whether the month you mean was actually a leap month that year.
How the lunar month and leap month are determined
New moon time is computed from orbital mechanics formulas (not a fixed table); each lunar month starts on the day of a new moon; a leap month is inserted in a lunar year that contains 13 new moons between two winter solstices, placed at the first month lacking a principal solar term.
- New moon = the moment the moon is between Earth and the sun, computed to sub-day precision using a periodic astronomical formula
- Leap month (nhuận) = an inserted 13th lunar month in certain years to keep the lunar calendar aligned with the solar year over time; the specific month that repeats varies by year
- Time zone = UTC+7 (Vietnam), the reference used for all calculations on this tool
Example date conversions
| Solar date | Lunar date | Note |
|---|
| 2024-02-10 | 1/1/2024 | Vietnamese Tet (Year of the Dragon) |
| 2025-01-29 | 1/1/2025 | Vietnamese Tet (Year of the Snake) |
| 2020-05-23 | 1/4/2020 (leap) | First day of the leap 4th lunar month in 2020 |
| 2000-01-01 | 25/11/1999 | Late in the lunar year before Tet 2000 |
Frequently asked questions
Why does this calculator use UTC+7 instead of China's UTC+8?
The lunar calendar is computed from the moment of each new moon relative to local midnight, so the same astronomical new moon can fall into a different calendar day depending on time zone; Vietnam's official calendar uses UTC+7, so in most years this matches the Chinese lunar calendar (UTC+8) exactly, but in a handful of boundary years the two differ by one day or, rarely, an entire month.
What is a leap month, and why do some years have one?
The lunar calendar's 12 months (each starting at a new moon) add up to about 354 days, roughly 11 days short of the 365-day solar year, so every two to three years an extra 13th month is inserted to keep lunar New Year from drifting far from late winter; the leap month takes the number of the preceding regular month, so a leap month 4 comes right after month 4.
Why do I need to specify whether a lunar month is a leap month when converting Lunar to Solar?
In a year with a leap month, two different periods of the solar calendar share the same lunar month number, so entering just a day and month is ambiguous; checking the leap-month box tells the converter which of the two you mean.
How accurate is this for very old or far-future dates?
This tool has been verified against 1900-2100 using the same formulas published for Vietnam's official lunar calendar and cross-checked with round-trip conversion and known Tet dates; results outside this 1900-2100 range are not offered, since astronomical calculations accumulate more uncertainty further from the present.
This calculator is verified for the years 1900 through 2100 and calculates the Vietnamese lunar calendar (UTC+7); it does not implement China's UTC+8 or Korea's UTC+9 official variants, which can differ by a day or, in rare boundary years, by a full month.