How to chart a lunar leap month

The lunisolar calendar sets its months by the phases of the moon: twelve synodic months come to roughly 354 days, which is about eleven days short of the solar year (roughly 365.24 days). Chinese calendars therefore insert leap months under the “seven leap months in nineteen years” rule, so that the calendar keeps pace with the four seasons. A lunar leap fourth month, for instance, follows the fourth month directly — but it does not produce a thirteenth stem-and-branch pillar.

In BaZi, the month pillar is strictly divided by the twelve “section” solar terms: the Start of Spring opens the tiger month, the Awakening of Insects opens the rabbit month, the Pure Brightness opens the dragon month, the Start of Summer opens the snake month, the Grain in Ear opens the horse month, the Minor Heat opens the goat month, the Start of Autumn opens the monkey month, the White Dew opens the rooster month, the Cold Dew opens the dog month, the Start of Winter opens the pig month, the Major Snow opens the rat month and the Minor Cold opens the ox month. The switch has nothing to do with the first day of a lunar month.

The most common mistake in folk charting is to treat “leap fourth month” as the snake month, or to mistake a leap month for the following month. The month pillar carries the month command — the outline of the chart — so getting it wrong skews the whole reading: whether the Day Master has the month, the strength reading, the configuration and the arrangement of the Ten Gods.

Step one: confirm the birth year, month and day, and whether it is a leap month. Take 2025 as an example: the ordinary lunar sixth month and the “leap sixth month” are a full month apart on the solar calendar, so if the leap-month option is missed the converted date slips by 29 to 30 days.

Step two: use a high-precision perpetual calendar to convert the lunar leap-month date into a standard solar date and time (year, month, day and hour).

Step three: check the solar-term boundary against that solar moment. For a “leap fourth month”: if the converted solar moment has not yet reached “Grain in Ear”（芒种, usually on 5 or 6 June）, the month pillar is still the snake month; if it has passed, the month pillar has formally entered the horse month. So people born in a leap fourth month may fall in either the snake month or the horse month, depending entirely on the exact solar-term moment.

Step four: the birth hour still follows the usual two hours per branch. A leap month changes where the solar date lands, not how hours are divided within a day.

In household registers, genealogies and older relatives' retelling, a birthday is often recorded purely in the lunar form, such as “the sixth day of the leap fifth month”. When that happens, do not quietly change it to May or June on your own.

In the 问真岛 charting tool, choose the lunar calendar and then pick the month marked “leap” in the month dropdown (for example “leap fifth month”); the system converts it to the exact solar moment using an astronomical perpetual calendar and charts from there.

If you use a simpler tool that does not support leap months, first look up the exact solar date for that leap-month day in a professional perpetual calendar, then enter it in solar-calendar mode.

First, check the year pillar: if you were born in a leap month or the first month around the lunar new year, the year pillar has to be judged by the solar Start of Spring — never by the Spring Festival on the first day of the first lunar month.

Second, check the month pillar: confirm that the branch of your month pillar comes from the solar-term interval your birth moment falls in, rather than from mechanically applying the lunar month name. The lunar “fifth month” is not automatically the horse month, and the “leap fifth month” certainly cannot be decided as the goat month or the horse month by opinion.

As long as those two are converted correctly by solar term, the day and hour pillars can be read by the standard rules. A leap month is only a mechanism for reconciling the calendar; it carries no special 命理 label, and there is no need to believe the fringe sayings about a “leap-month fate”.