Free tool
Lunar-solar converter
Converting between lunar and solar dates has to handle the leap month, a repeated month that keeps the lunar year in step with the solar one. Whether a year has one, and which month repeats, depends on the timezone used, so Vietnamese and Chinese calendars place it in different months some years. The same solar date then maps to different lunar dates.
Convert both ways between the lunar and solar calendars with the day and year pillars and the solar term, handling leap months correctly, and measure the span between any two dates.
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Why is lunar-to-solar harder than the other direction?
The solar calendar counts Earth's orbit around the Sun: month lengths are fixed, and only February shifts with leap years. The Vietnamese lunar calendar follows the moon instead, each month begins on the new moon, the moment the Moon passes between Earth and Sun, so a month runs 29 or 30 days according to the actual cycle, with no alternating rule. Twelve lunations come to about 354 days, roughly 11 short of a solar year, so a leap month is inserted every few years to stop Tết drifting into summer.
That is why the lunar-to-solar direction needs a question the other does not. One solar date maps to exactly one lunar date. But in a year with, say, a leap fourth month, "the 15th of the fourth lunar month" occurs TWICE, once in the regular month, once in the leap month, landing on two solar dates about a month apart. No tool can guess which you mean, so this page asks outright with the "leap month" checkbox; leaving it unticked takes the regular month.
The lunar calendar used in Vietnam is computed for UTC+7, while the Chinese calendar uses UTC+8. When a new moon or a solar-term boundary falls near midnight, one hour is enough to push it into a different day, so some dates differ by exactly one day between the two, and in some years the two countries keep Tết on different dates. This page computes for Vietnamese time.
Results carry the day and year stem-branch pillars, because that is what most people reach for next: picking a date, checking an age, or matching a printed almanac. The solar term is shown too, the twenty-four terms are set by the Sun's position rather than the Moon's, so they are the solar element living inside a lunar almanac.
The third mode measures the gap between two solar dates in days, weeks and approximate months. "Approximate" is deliberate: solar months run 28 to 31 days, so no division yields an exactly correct month count, and a tool that prints "3.2 months" without saying it is rounding claims more precision than it has.
| Point of comparison | Solar calendar | Vietnamese lunar calendar |
|---|---|---|
| Basis | Earth's orbit around the Sun | Lunations; months start on the new moon |
| Days per month | 28–31 by fixed rule | 29 or 30, following the real lunar cycle |
| Days per year | 365 or 366 | 354–355; 383–385 in a leap year |
| How the drift is corrected | One extra day, 29 February | A whole extra month |
| How many dates one maps to | Always exactly one lunar date | A leap-month date maps to two solar dates |
| Time zone used | Not time-zone dependent | UTC+7 for Vietnam, UTC+8 for China |
Frequently asked questions
Why is there a "leap month" checkbox?
Because in a lunar leap year that month occurs twice, so a date like "the 15th of the fourth month" maps to two different solar dates. Nothing can guess whether you mean the regular or the leap month, so the page asks. Unticked means the regular month. The solar-to-lunar direction needs no such question, since each solar date has exactly one lunar counterpart.
Why does the Vietnamese lunar calendar sometimes differ from the Chinese one by a day?
Because the two are computed for different time zones: Vietnam uses UTC+7, China UTC+8. The new moon is an astronomical instant with a specific time of day; when it falls close to midnight, one hour is enough to place it on the next day in one zone but not the other. That is also why the two countries occasionally keep Tết on different dates.
How often does the lunar calendar add a leap month?
Roughly every two to three years, precisely seven times in every nineteen. Twelve lunations come to about 354 days, some 11 short of a solar year; without correction Tết would drift into summer within a few decades. The leap month repeats the number of the month before it, so a leap fourth month is the year's second fourth month.
Are the solar terms lunar or solar?
Solar, even though they are printed in a lunar almanac. The twenty-four terms divide the Sun's position along the ecliptic at 15-degree intervals, so they sit almost fixed against solar dates, Lập Xuân always near 4 February, the winter solstice near 21 December. They are the solar component inside a lunar system.
Why is the month figure in the span mode marked approximate?
Because solar months run 28 to 31 days, no division turns a day count into an exact month count. The day and week figures are exact; the month figure is an estimate for a sense of scale, and the page says so rather than printing a number that looks precisely computed.
Is conversion reliable for dates before 1900?
Treat distant dates with care. The computation is astronomical and so extends far back in principle, but the calendar as actually promulgated depended on the conventions of each dynasty and era, which did not always match pure astronomy. For dates within the last several decades the result is dependable.
What is the day pillar used for?
The day pillar is that day's heavenly stem and earthly branch, repeating on a 60-day cycle. It feeds most traditional date selection: auspicious hours, choosing a day for a task, wedding dates, groundbreaking dates. The page therefore returns the day and year pillars alongside the conversion, so you need not look them up separately.