Check Japanese age and dates with the rules that actually apply

koyomi-time explains Japanese ways of counting age, school year, and era dates — from statute and from practice. Calculations stay on your device; we do not send birth dates to a server.
Age calculator
Legal age, kazoedoshi, yakudoshi, next birthday
Age chart
Man-nenrei, wareki, and eto by birth year
Era converter
Includes dual labels in era-change years
School-year chart
CV text and early-birthday (hayaumare) support
Days between
Difference of two dates, optional start-day count
Date calculator
Add or subtract days, months, or years
Yakudoshi
Mae, hon, and ato (kazoedoshi)
Eto (zodiac)
Stems, branches, 60-year cycle
Longevity
Kanreki, koki, kiju, beiju, and more
Days alive
Day 100, 1,000, and 10,000
Birthday countdown
Days until the next birthday
Born in …
Birth years 1900–2026
Age → birth year
Work backwards from age to birth year
Gregorian year = era start year + era year − 1. Example: Showa 50 is 1926 + 50 − 1 = 1975. In an era-change year the same Gregorian year has two era labels (1989 is Showa 64 and Heisei 1; 2019 is Heisei 31 and Reiwa 1). Before 1 January of Meiji 6 Japan used a lunisolar calendar, so era years do not map simply to Gregorian years.
EraGregorianEra yearsEra start date
Reiwa2019–1–1 May 2019
Heisei1989–20191–318 January 1989
Showa1926–19891–6425 December 1926
Taisho1912–19261–1530 July 1912
Meiji1868–19121–4523 October 1868

Legal age increases the day before the birthday

Man-nenrei (満年齢) follows Japan’s Age Calculation Act: you turn a year older at the end of the day before your birthday. Someone born on 2 April turns a year older at the end of 1 April.
Government, insurance, and many applications use this “day-before” rule. It is not the same instant as the calendar birthday itself.

The school-year cutoff is 2 April

Japanese schools assign grade by legal age on 1 April. Because of the day-before rule, the practical cohort boundary is being born on 2 April.
A 1 April birthday is usually the previous grade; a 2 April birthday is the next. Use that cutoff when you check enrolment or promotion.

Man-nenrei and kazoedoshi

Man-nenrei is the usual modern age: counted from birth, increasing the day before the birthday. Kazoedoshi (数え年) starts at 1 in the birth year and increases every New Year.
Shrine yakudoshi and some customs still use kazoedoshi. Mixing the two can shift the number by one or two years, so pick the count that matches the purpose.

Frequently asked questions

When does legal age increase?

Under the Age Calculation Act, man-nenrei increases at the end of the day before the birthday. For a 10 May birthday, you turn a year older at the end of 9 May.

Why is the school-year boundary 2 April?

Schools use legal age on 1 April. With the day-before rule, 1 April and 2 April birthdays fall in different grades, so the practical cutoff is 2 April.

What is the difference between kazoedoshi and man-nenrei?

Man-nenrei is current legal age (increases the day before the birthday). Kazoedoshi starts at 1 in the birth year and increases on 1 January. Yakudoshi often uses kazoedoshi.

Are birth dates sent to a server?

No. Age and date calculations run in the browser. We do not send birth dates to a server in order to calculate. See the privacy policy for details.