Korean lunar birthdays on the solar calendar, and leap months

Lunar vs solar calendar

The solar calendar counts a year by the sun. The Korean lunar calendar counts a month by the moon. One lunar cycle takes about 29.5 days, so a lunar month has 29 or 30 days, called short and long months.

Twelve lunar months come to about 354 days, roughly 11 days short of a solar year. To keep the seasons in place, an extra month is added every few years. That is the leap month (yundal). It appears about seven times in 19 years and takes the name of the month before it, like leap month 4.

So one lunar date lands on a different solar date each year. Lunar 3/15 this year and next year can be more than ten days apart on the solar calendar.

Why families look it up every year

Many Korean parents and grandparents keep their birthday on the lunar calendar, and many families record a memorial day for jesa the same way. Because the date isn't fixed on the solar calendar, people check it again every year.

Phone calendars repeat events by solar date only. Put in lunar 3/15 once and set it to repeat, and next year it shows on the same solar day, which is wrong. That is why this tool writes one event per year instead.

How this tool calculates

It asks only for the lunar month, day, and whether it is a leap month. No birth year. Dates come from korean-lunar-calendar, a table that follows the KASI standard, and cover lunar dates from 1 January 1900 to 18 November 2050. Outside that we say we can't confirm it and don't guess.

You see every date that falls in this solar year and next. Lunar months 11 and 12 spill into January and February of the next solar year, so the tool checks the lunar years on either side. A date in early lunar month 12 can land twice in one solar year, or not at all.

The calendar file holds one event per year from today through 2050. Reminders are alarms inside the file, rung by your phone. What you enter stays on the screen, and Save keeps it only in this browser.

We pulled 37 conversions from KASI's own converter and matched them one by one against this tool, including ten leap-month years, a day 30 in a short month, and the end of the range (18 November 2050).

Leap-month birthdays

A given leap month comes back only every few years. In years without it, the widely used custom is the same day of the regular month, and that is what this tool shows.

Years that do have the leap month vary by family. The Encyclopedia of Korean Culture records families on Jeju holding the rite twice, in the regular month and the leap month, and says leap-month birthdays are treated the same way. Some private converters use only the leap month in years that have it. So you pick from three: regular month only, both, or leap month only.

Some families switched to the solar birthday. If yours did, use the solar date directly.

Born on day 30

If the date is lunar day 30, some years that month ends on day 29. We found no written rule for this. Some families use day 29, some use the day after.

The tool works out both and builds the calendar file from the one you pick. In years where it moved, the result says so next to the date.

Common snags

Not sure if it's a leap month? Try without it first. It is a leap month only if the family record says so. If you know the birth year, enter the solar birthday under 'Solar date to lunar date' and it will tell you.

If a date is a month off from another calendar, check the leap month setting. Entering a regular month as leap gives a date about a month later. If it still differs, compare with KASI's converter.

Families hold jesa at different times. This tool gives the date only.