日付の差、カウントダウン、年齢、時間計算ツール。
暦上の日数・複数単位の年齢・経過時間が必要なときに使うカテゴリーです（暗算の代替には向きません）。D-Day と日付差は別の問いに答えます（単一目標までのカウントダウン vs 二つの日付のあいだ）。取り違えやすい点です。年齢計算機は専用ページで地域の年齢の数え方を反映する場合があるため、どの規則か確認してください。結果は入力した日付から一意に決まり、ページで明示しない限り営業日・祝日・契約条項は織り込みません。
各計算機の用途・前提・よくある誤りを段落でまとめました。結果を読む前に前提を確認してください。
Find your exact age in years, months, and days — accurate to the day. Enter your date of birth and optionally choose a custom "as of" date to calculate age at any point in time, past or future. Leap years are handled automatically. Includes Korean man-nai (만 나이) for Korean administrative contexts.
The D-Day Calculator counts exactly how many days remain until a future event — or how many days have passed since a past date. The name comes from military usage where "D" marks a designated target date, but in everyday use it is most commonly used for personal and professional countdowns: exam dates, travel departures, product launches, anniversaries, visa expiry dates, payment due dates, and project deadlines. Unlike the Date Difference Calculator (which compares two arbitrary dates you specify), a D-Day calculator is oriented around today — it automatically answers "how many days until or since [event]?" You can also set a custom start date if you need to count from a date other than today.
The Date Difference Calculator answers one deceptively simple question: exactly how far apart are two dates? Enter a start date and an end date to get the precise gap in days, weeks, months, and years. This is most useful when the exact count matters — for project deadlines where contract language specifies "30 calendar days," for lease expiry calculations where one day off has legal consequences, for loan term verification, for checking how long until a visa or passport expires, or for confirming probationary periods and notice periods at work. The month count uses actual calendar months, not 30-day approximations — so the gap from January 1 to March 1 is exactly 2 months regardless of how many days are in January and February. A business days option (excluding weekends) is available for work-related calculations.
The Time Duration Calculator finds the exact duration between any two times of day — down to the second. Enter a start time and end time to get the elapsed time in hours, minutes, and seconds. You can also add a duration to a start time to find the end time, or sum multiple time intervals together for total time tracking. The calculator handles overnight durations (crossing midnight) automatically. Ideal for tracking work sessions, timing events, calculating billing increments, cooking timers, and travel time planning.
日付差計算機で開始日と終了日を入力すると、日数・週数・月数が表示されます。
年齢計算機で年・月・日に加え、誕生日からの総日数も表示できます。
目標日を入力すると、その日まで（または経過後）の正確な日数を数えます。
はい。うるう年・各月の日数・夏時間の扱いを自動反映します。