🌙 Japanese Lunar Date Converter (旧暦 & Moon Age)

Before Japan adopted the Gregorian calendar in Meiji 6 (1873), the country used a lunisolar calendar known as 旧暦 (kyureki, "old calendar"). Like the Chinese calendar it was based on, each month followed the cycle of the moon: a new month began at the new moon, and the full moon fell on the 15th day. To keep the calendar aligned with the solar seasons, a leap month (閏月, uruuzuki) was inserted roughly every three years.

Even after Japan switched to the Western calendar, the old calendar survived in many cultural contexts:

• Rokuyō (六曜) — the six-day fortune calendar used for choosing wedding dates (see our rokuyō checker)
• Setsubun (節分) — the day before the start of spring in the old calendar
• Tsukimi (月見) — moon-viewing festivals held on the old 8th month 15th night (十五夜)
• Traditional festivals — many regional matsuri still follow kyureki dates

The moon age (月齢, getsurei) counts the number of days since the last new moon, from 0.0 to 29.5. At 月齢 0 it's a new moon, at 月齢 7.4 it's a first-quarter moon, at 月齢 14.8 it's a full moon, and at 月齢 22.1 it's a last-quarter moon. Moon age is still printed in Japanese calendars and almanacs (暦, koyomi) today.

While the Western calendar is universal in modern Japan, understanding the old lunar calendar is essential for appreciating traditional literature, festivals, and the rhythm of the seasons. To learn the traditional month names (睦月, 如月…), see our month names guide.

Converting a Western date to the old lunisolar calendar is complex because the lunar month does not divide evenly into the solar year. The traditional calendar uses these rules:

• Each lunar month begins at the astronomical new moon and lasts 29 or 30 days
• Twelve lunar months total about 354 days — 11 short of the solar year
• To realign with the seasons, a leap month (閏月) is added about 7 times every 19 years (the Metonic cycle)

Because the exact new moon times vary, the conversion requires astronomical calculations. This tool uses a precomputed lunisolar table (based on the same algorithm as the Chinese lunar calendar) to find the old-calendar month and day for any Western date from 1900 to 2050.

The moon age is calculated from the last new moon using the synodic month of 29.53059 days. The result tells you how much of the moon is illuminated: 0 = completely dark, 14.8 = full, 29.5 = almost new again. This is the same information used for the rokuyō calendar — if you want the rokuyō (大安, 仏滅 etc.) for a specific date, use our rokuyō checker which combines the lunar date with the fortune calculation.

1. Type a date in Western format (e.g. 2026-09-15).
2. Click Convert to see the old Japanese lunar calendar date.
3. The result also shows the moon age (月齢) and phase name.
4. Use the moon age to plan moon-viewing events (tsukimi).
5. Compare with the rokuyo calendar for traditional fortune dates.

What is the old Japanese calendar (旧暦)?

旧暦 (kyureki) is the lunisolar calendar Japan used before switching to the Gregorian calendar in Meiji 6 (1873). Each month began at the new moon, the full moon was on the 15th, and a leap month was inserted every ~3 years to stay aligned with the solar seasons.

Why are traditional festivals on different Western dates each year?

Because they follow the old lunar calendar. 中秋の名月 (harvest moon) is always 旧暦8月15日, but that maps to a different Western date each year — usually mid-September to early October. The same applies to Setsubun, tsukimi, and many shrine festivals.

What does moon age (月齢) mean?

Moon age counts the days since the last new moon, from 0.0 to 29.5. 月齢 0 = new moon, 月齢 7.4 = first quarter, 月齢 14.8 = full moon, 月齢 22.1 = last quarter. It tells you how much of the moon is visible.

When was a leap month (閏月) last used in Japan?

The old calendar had leap months roughly every 3 years. Since Japan switched to the Gregorian calendar in 1873, leap months only appear in the traditional/Chinese calendar, not in daily life. They still affect the timing of lunar-calendar festivals.

Is this the same as the Chinese lunar calendar?

Yes, Japan's 旧暦 was based on the same lunisolar system as the Chinese calendar. After Japan adopted the Gregorian calendar in 1873, the traditional calendar was no longer officially used but survived in cultural and ritual contexts.

How accurate is this converter?

The lunisolar conversion uses the same astronomical algorithm as the Chinese lunar calendar, accurate from 1900 to 2050. Moon age is calculated from the synodic month (29.53059 days). For festival planning, always double-check with a published Japanese almanac (暦).

The full moon always falls on the 15th night (十五夜, jūgoya) of the old calendar month. The most famous is the 8th month 15th night (旧暦8月15日), known as 中秋の名月 (Chūshū no Meigetsu), the harvest moon. Moon-viewing (月見, tsukimi) is held on this date, usually falling in September or early October in the Western calendar. There's also a "13th night" (十三夜) in the 9th month — viewing both is called "both moons" (両月見).

If you see a date like "旧暦8月15日" in a traditional event announcement, it means the lunar date, but the event itself will be held on the corresponding Western date that year. Always check both. For the setsubun/eho direction, the calculation is based on the old calendar's division of the year into 24 solar terms (二十四節気).