When do you turn a year older? An administrative scrivener explains Article 143 of the Civil Code and the school year cutoff

“You turn a year older on your birthday”—is that what you think?
Actually, in legal terms, you turn a year older on the “day before.”
This concept is deeply involved in real life, including elections, driver's licenses, and school enrollment ages.
In this article, I will explain the legal rules of age calculation and their impact on practical matters from the perspective of an administrative scrivener.

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■ Article 143 of the Civil Code is the key

Article 143 of the Civil Code (Expiration of Period)
“When a period is fixed by years, months, or weeks, the period expires on the day preceding the corresponding day.”

What this provision means is,
for example, a person born on April 1, 2000, turns 25 at midnight on March 31, 2025.
In other words, the legal treatment is that you have already reached your new age on the day of your birthday.

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■ What about people born on a leap day (February 29)?

February 29 only comes once every four years, but it is not the case that you don't turn a year older because the day doesn't arrive.

In non-leap years, you turn a year older at midnight on February 28.

This is because when applying the “day before” principle to a year where February 29 does not exist, February 28 becomes the timing for the age increase.

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■ The reason the school year cutoff is April 2

Actually, this “turning a year older on the day before” rule is also related to the school year cutoff in Japan.

The age requirement for elementary school enrollment is that “the day you reach the age of 6 must be before the start of that school year.”

Applying the Civil Code rules:
• Born April 1 → Turn 6 at midnight on March 31 → Enroll that year (early birth)
• Born April 2 → Turn 6 at midnight on April 1 → Enroll the following year

In short, the school year cutoff is April 2 because ages are counted in accordance with Article 143 of the Civil Code.

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■ Points to note in practice

Because this “day before” addition is surprisingly little known, it is important in the work of an administrative scrivener to avoid the following mistakes:
• Driver's license: Application is possible from midnight on the day before, not on the birthday itself
• Right to vote: You can vote if you have reached 18 by midnight on the day before
• Contracts and permits: Misreading age requirements can lead to applications being rejected

■ Conclusion

A difference of just one day can change the timing of your life.
That is the world of law.

If you are ever unsure about how birthdays or ages are handled, please remember “Article 143 of the Civil Code.”

Article 143 of the Civil Code (Expiration of Period)
“When a period is fixed by years, months, or weeks, the period expires on the day preceding the corresponding day.”

And when you are anxious about practical matters or procedures, consulting a professional can help prevent trouble before it happens.