An inserted month follows a calendar’s rules
HKO’s solar-term overview explains Chinese intercalation by relating lunar months to the major solar terms. An inserted month preserves the calendar’s alignment with the seasonal year. This overview is useful orientation; use the named calendar’s full rules or published tables to determine an actual leap month.
A leap month is not a Gregorian leap day. It is also not the repeated-day notation found in Tibetan calendar methods. The selected calendar determines how an extra month is numbered and where it occurs.
Worked example · the repeated second month in 2023
In HKO’s March 2023 almanac, 21 March is ordinary second month, day 30. On 22 March the date becomes leap second month, day 1. The month number is still two; the leap marker makes the second occurrence unambiguous.
Try these dates in the Atlas date-only lookup and compare the month, day and leap flag together. Copying only “month 2” loses information needed to reproduce the date.
Preserve the marker in records and reports
When reading a Chinese-calendar result, keep the lunar year, month number, day and leap flag as one date. Record its Gregorian reference date too. Do not turn the second occurrence of a numbered month into a guessed new month number.
Atlas currently accepts Gregorian birth dates for this workflow. A family record written only as a traditional calendar date needs a supported conversion and its stated leap convention before it can be entered reliably. This page does not add a reverse calendar converter or decide birthday-observance customs.
Do not transfer an ordering rule between traditions
Use the Chinese-calendar explorer for its declared modern Chinese reference. Use the separate Tibetan tool only under its own accepted lineage and month-ordering contract. The shared English phrase leap month does not make their calculations interchangeable.
The glossary’s etymology describes the English terminology; it is separate from the astronomical or calendrical rule that produces an extra month. No mythology or historical word origin is inferred from a repeated number.
Word origin · etymology
Descriptive English calendar phrase; intercalary is a Latin-derived synonym.
Leap month is the English label for an inserted calendar month; its exact coinage is not established by these sources. Intercalary comes from Latin intercalarius, related to intercalare, inserting a day or month into a calendar.
Merriam-Webster · intercalary, word history ↗Merriam-Webster · intercalate, word history ↗
How we know · epistemology
Calendar intercalation rule
An extra month follows the selected calendar’s alignment and numbering rules. In the Phugpa method implemented here the leap occurrence precedes the regular month of the same number; that ordering must not be transferred automatically to another tradition.
Hong Kong Observatory: the 24 solar terms ↗Svante Janson · Tibetan calendar mathematics ↗
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