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年齢計算機

生年月日から今日または指定日までの年・月・日数を数えます。それより前の日付は受け付けません。うるう年はカレンダーどおりに扱います。

ツールを読み込み中…

このツールはブラウザ内で動作します。入力内容、ファイル、生成した値はこの端末に残ります。お気に入りと最近使ったツールを利用する場合も、ローカルストレージに保存されるのはツール名だけで、パスワード、文書、QR コードの内容は保存されません。 詳しくはプライバシーポリシーをご覧ください。

このツールの詳しい解説（使い方、例、よくある質問）は、現在英語のみでご用意しています。

What this tool does

The Age Calculator counts whole years, then the remaining months, then the remaining days from a date of birth to today, or to another date you pick. It also shows the total number of days. Use it for a birthday, an anniversary, or the age someone will be on a future day. It follows the calendar, including leap days. It does not divide the day count by 365.

How to use

  1. Enter the date of birth.
  2. Leave the second date empty to use today, or pick another date if you want the age on that day.
  3. Press Calculate. The page shows years, months, days, and total days.
  4. If the second date is earlier than the birthday, the page stops and asks for a later date.

Examples

  • Born 1 January 2000, age on 1 January 2026

    Enter 2000-01-01 as the date of birth and 2026-01-01 as the other date. Calculate shows 26 years, 0 months, and 0 days. Total days is 9,497. That day count includes the seven leap days from 2000 through 2024. Dividing 9,497 by 365 does not produce this age.

  • A leap-day birthday

    Enter 2000-02-29 and 2001-02-28. Calculate shows 0 years, 11 months, and 30 days. Total days is 365. The year 2001 is not a leap year, so 29 February is not a date in 2001. The count steps back one month and uses the length of January 2001, which is 31 days.

  • Age on a future date

    Enter 2010-06-15 and 2030-06-15. Calculate shows 20 years, 0 months, and 0 days. Total days is 7,305. A date before the birthday is rejected with the message: The selected date cannot be before the date of birth.

How it works

Age is counted from the calendar date of birth to the selected date. The tool subtracts years, then months, then days. If the day number goes negative, it borrows the length of the previous month. If the month number goes negative, it borrows a year. Total days is the number of midnights between the two dates in UTC, so a leap day in the span is one extra day.

Why dividing by 365 is a different number

From 1 January 2000 to 1 January 2026 is 9,497 days. 9,497 ÷ 365 is about 26.02, which is not “26 years, 0 months, 0 days.” The calendar age is an anniversary count. The day count is a separate line on the page.

A year in this tool is not 365 days and not 365.25 days. It is the same month and day in a later year, adjusted when that day does not exist. That is why a 29 February birthday does not turn one year old on 28 February of the next year.

How a short month is borrowed

If the “as of” day is earlier in the month than the birth day, the tool takes one month back and adds the number of days in the previous month. January borrows December of the previous year, which has 31 days.

An impossible date, such as 31 April or 29 February 2001, is rejected before any age is shown. The birthday field must be a real calendar date.

What this count is not

The result is calendar arithmetic. It is not a medical age, and it is not a legal age for a contract, a license, or a benefit. Those rules sometimes count the birthday itself in a special way. This page does not.

Leaving the second date empty uses the date in your browser, based on the device clock. Changing the clock changes “today.” A date you type is used as you typed it.

Privacy

Both dates stay in this tab. The count does not send them to a server and does not write them to local storage. Analytics, if you accept it, can record that the page was opened. It does not receive the dates.

Tips

  • Use the second date when you need an age on a birthday, a future appointment, or a past anniversary. Empty means today.
  • For someone born on 29 February, pick a real date in a non-leap year, such as 28 February or 1 March. The page will not invent 29 February 2001.
  • Read years, months, and days together. The total-days line is the length of the span, not a second age.
  • If the result looks a month off, check which date is the birthday. A reversed pair is rejected when the second date is earlier, and it is a different age when both dates are valid.

Limitations

The count runs from the birth date to today, or to another date you choose. A date earlier than the birthday is rejected. Impossible calendar dates are rejected. This is calendar arithmetic, not a medical age or a legal age for a contract.

See the disclaimer for what these tools do not cover.

FAQ

How do I calculate my exact age?
Enter your date of birth and press Calculate. The page counts full years, then full months since the last birthday month, then the remaining days. It also shows total days. Leaving the second date empty uses today.
How is age counted for someone born on February 29?
The count follows the real calendar. From 29 February 2000 to 28 February 2001 the result is 0 years, 11 months, and 30 days, and the total is 365 days. The tool does not treat every year as 365 days.
Can I calculate age on a future date?
Yes, as long as that date is not before the date of birth. From 15 June 2010 to 15 June 2030 the result is 20 years, 0 months, and 0 days, with 7,305 total days.
Why is the total days not years times 365?
Total days includes leap days that fall inside the span. From 1 January 2000 to 1 January 2026 the calendar age is 26 years, 0 months, and 0 days, and the total is 9,497 days. Dividing that total by 365 is a different calculation.
What if the second date is before the birthday?
The page rejects it. The message is: The selected date cannot be before the date of birth.
Is my input sent to a server?
No. The dates stay in this tab while the age is counted. They are not sent to a server or written to local storage.

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