What is a lunar leap month, and why most calendar apps get it wrong
The Chinese lunar calendar has twelve months of 29 or 30 days each. Twelve such months only add up to roughly 354 days — eleven days shorter than the solar year. Left alone, the seasons would drift through the calendar (Lunar New Year would slide into summer within a couple of decades). To stop the drift, the lunar calendar inserts an extra month every two to three years. That extra month is called a leap month, or rùnyuè (闰月).
In any nineteen-year window there are seven leap months — that's the Metonic cycle, the same arithmetic the Hebrew and Buddhist calendars use. The leap month is always a duplicate of the month before it: there's a regular fifth month, and then a leap fifth month directly after, before the regular sixth month begins.
Why this is a problem for calendar apps
Most calendar apps that say they support a lunar repeat actually only support a normal lunar month — they assume every month exists every year. That works fine for the vast majority of dates, but it breaks for people born in a leap month. Here's the worst case: someone born on the eighth day of the leap fifth month in 1990. Their actual lunar birthday only exists in years that also have a leap fifth month. Looking at the next few decades, those are 1990, 2009, 2028, 2047, 2066 — five times in seventy-six years.
So what should happen in 2026, when there is no leap fifth month at all? That's the question every lunar calendar tool has to answer, and most just silently pick a strategy without telling you.
The three reasonable strategies
There is no universally correct answer here — different families celebrate differently. MoonCal lets you pick one of three strategies when you create a lunar event in a leap month:
| Strategy | What MoonCal does in a year with no matching leap month |
|---|---|
| Skip | No event that year. The next occurrence will be the next leap-five-month year. |
| Use the same regular month | Use the regular fifth month, same day. This is the most common choice — many families celebrate on the regular month in non-leap years. |
| Use the previous month | Use the regular fourth month, same day. Less common, but used in some families. |
A concrete example: 1990 leap-five-month birthdays in 2026
Say someone was born on the eighth day of the leap fifth month in 1990 — that maps to 1990-06-30 on the Gregorian calendar. In 2026 there is no leap fifth month at all (the next leap month in 2026 is the leap sixth month, but the rules don't allow that as a substitute). With each MoonCal strategy:
All three are arithmetically valid. Pick the one that matches what your family actually does.
How widely-used calendar apps handle this
Without naming names, here's what we've observed when testing major calendar apps and ICS feed providers:
This is the kind of detail that doesn't matter for 95% of users — but if you or someone you love is in that 5%, it's the difference between a birthday that quietly disappears and one that lands on the day your family actually celebrates.