The Korean lunisolar calendar

Korean date records

Korean calendars commonly place a Gregorian date beside a date from the traditional lunisolar calendar. The Gregorian date locates an occasion in the civil year, while the lunar date preserves its position in the traditional sequence of months. Lunar dating remains important for festivals, birthdays and family anniversaries.

Two Korean labels are particularly useful. Yangnyeok (양력), literally “solar calendar,” normally identifies the Gregorian date in a modern record. Eumnyeok (음력), “lunar calendar,” identifies the traditional date. The latter calendar is structurally lunisolar because leap months keep its lunar months aligned with the seasons. Pyeongdal (평달) means an ordinary month, and yundal (윤달) means a leap month.

A lunar month begins with the civil day containing the astronomical new moon. Its dates run consecutively through a month of 29 or 30 days. An ordinary year has twelve lunar months, while a year with a leap month has thirteen. The leap month repeats the preceding month's number and has its own sequence of days.

Day 15 is a numbered calendar day, not a guarantee that the Moon reaches exact fullness during that evening. The civil day containing the new moon becomes day 1 regardless of the hour of conjunction, and the interval from new moon to full moon varies.

Astronomical reference

Modern South Korean calendar calculation uses UTC+9, associated with the meridian 135° east. The Chinese calendar uses a reference time one hour earlier. When a new moon receives different civil dates under these conventions, the calendars can begin a month on different days.

Month numbering also depends on the Sun. The month containing the winter solstice is month 11, and the placement of the twelve principal terms helps determine leap months. A principal term is one of the solar markers spaced at 30-degree intervals along the Sun's apparent annual path. A change in which month contains a principal term can therefore affect the month number as well as the day number.

Korean and Chinese dates in 2012

In the Korean calendar, leap month 3 came before ordinary month 4. In the Chinese calendar, ordinary month 4 came first and was then repeated. The same written date, ordinary month 4, day 8, consequently identified days thirty days apart.

This difference cannot be corrected by adding an hour to an already converted lunar date. Reference time determines how the astronomical events are assigned to civil days before month boundaries and numbers are established.

Date conversion

A usable Korean date record includes the original year designation, calendar label, month, day and leap-month status. Without a calendar label, a birthday's month and day may refer to either Gregorian or lunar dating. Without a leap marker, a lunar date may identify either of two months in the same year.

The Korean solar–lunar conversion service provides separate choices for an ordinary month, a leap month and an unknown designation. Enter the complete original date and retain it alongside the converted result. When a historical lookup supplies both Gregorian and Julian dates, record which civil calendar you use for the equivalent.

Korea adopted Gregorian civil dating in 1896, but the lunar calendar continued in family and festival use. Historical almanacs also used reference meridians and time conventions that differed from today's. A date in an older record therefore needs a historical Korean reference rather than a modern Chinese conversion.

A converted birth date and the rule for celebrating later birthdays are separate matters. The first identifies the day of birth. The second depends on family practice, especially when the birthday belongs to a leap month that does not occur in the anniversary year. Record that observance rule separately from the original date.

Worked example: finding a Korean lunar birthday

Suppose you know someone’s ordinary Gregorian birth date and want to find their Korean lunar birthday in 2027. Use the official KASI converter to identify the original lunar date first, then search for its anniversary.

Start with the birth date

Choose Gregorian-to-lunar (양력을 음력으로) and enter the complete original birth date, including the birth year—not 2027. Save the returned Korean lunar year, month and day, together with its ordinary (평달) or leap (윤달) designation. Keep this alongside the original Gregorian date.

Find the birthday in the chosen civil year

Switch to Find a specific lunar date (특정 음력일 찾기), the separate period-search tab. For this example, enter 2027 as both the starting and ending Gregorian year, then enter the saved lunar month, day and ordinary/leap designation.

Read the returned Gregorian date or dates for your reminder. Searching by Gregorian year avoids having to guess which lunar year contains an occurrence near January or February. Check the full returned dates rather than carrying forward last year’s Gregorian birthday.

When the original leap designation is unknown

If you instead have only a lunar birth record without an ordinary/leap designation, choose Lunar-to-Gregorian (음력을 양력으로). Enter the original lunar year, month and day, then select unknown (평달인지 윤달인지 알지 못함) rather than assuming an ordinary month.

Compare any candidate dates with reliable birth documentation. If both ordinary and leap possibilities remain unresolved, preserve both. For anniversary searches, run the period search separately for each designation; finding a match in the anniversary year does not establish which was the original birth date.

If the required leap month or day does not exist in the chosen year, keep that absence distinct from any family decision about when to celebrate. A substitute celebration date belongs in the reminder as a family choice, not as a correction to the birth record.