年齢計算（生年月日からの詳細計算）
生まれてから現在までの満年齢を、年・月・日単位で正確に割り出します。
Calculate an age
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仕組みと計算ルール
- Age is a calendar difference from the birth date to the “as of” date (today by default), computed by stepping years and months with the same clamping rules as the add-months engine.
- Alongside years/months/days, the tool shows the underlying totals: elapsed days (start exclusive), whole weeks, and total months.
- The next birthday is the birth date advanced by one more year under the same rules; for a Feb 29 birthday that is Feb 28 in non-leap years.
知っておくと便利なポイント
- Set the “as of” date to any past or future date for fixed-cutoff questions (eligibility, forms); it defaults to today but never has to be.
- The convention for Feb 29 birthdays is displayed with the result: in non-leap years the birthday is treated as Feb 28.
- A birth date in the future (relative to the as-of date) is reported as an error rather than a negative age.
関連ツール
よくある質問
How are February 29 birthdays handled?
By a stated rule: in non-leap years, a Feb 29 birthday is treated as occurring on February 28, so the age ticks over on Feb 28. The page displays this convention next to the result.
Why is my age in “total days” different from another site?
Most disagreements stem from counting conventions (inclusive vs exclusive) rather than math errors. This tool counts elapsed days since birth (start exclusive).
Can I calculate someone’s age as of a past date?
Yes, set the “as of” date instead of today. The result computes against that date, which is helpful for forms and eligibility cutoffs.