Age and age calculation

The age calculator determines a calendar-based age between a birth date and a freely selected comparison date. It is a date calculation without a birth time, not a biological, medical or legal classification of age.

The page uses a birth date and a comparison date. The comparison date starts as today’s local calendar date but may be any past or future date that is not before birth. With valid input it shows years, months and days, complete months, days, weeks and remaining days, derived hours/minutes/seconds, birthdays and progress through the current year of life.

The service uses plain `{ year, month, day }` values and an integer proleptic-Gregorian day number. It first finds the maximum number of complete calendar months directly from the original birth date, then derives years and remaining months and finally counts calendar days from that month anchor. It does not divide total days by 365 or by an average month length.

Total days exclude an extra count of the start date. Complete weeks are `floor(totalDays ÷ 7)` and remaining days are `totalDays mod 7`. Hours, minutes and seconds are mathematical conversions of calendar days; because no birth time is entered, they are not a live age measured to the second.

For 2000-02-29 → 2024-02-28, the checked service result is 23 years, 11 months and 30 days with 287 complete months. The next birthday is 2024-02-29 in one day, when age becomes 24. On 2024-02-29 it is exactly 24 years, 0 months and 0 days with 288 complete months. For 2000-09-20 → 2026-09-19 it is 25 years, 11 months and 30 days; on 2026-09-20 it is exactly 26 years.

The last birthday is the birthday already reached; the next birthday is today with zero days remaining when today is the birthday, otherwise the following calendar birthday. Progress is elapsed calendar days since the last birthday divided by the days to the following birthday, so a year of life can contain 365 or 366 days. It describes a position in that calendar interval only.

For a 29 February birth, this calculator observes 28 February as the birthday in a non-leap year. In a true leap year it remains 29 February. At month ends, the same calendar clamp is used: 2023-01-31 → 2023-02-28 is one complete month, while 2023-01-31 → 2023-03-30 is one month and 30 days.

Input must be a real `YYYY-MM-DD` ISO calendar date with at least four year digits; year 1 and safe integers form the supported year range. Invalid dates and a comparison before birth are rejected. There is no timezone conversion, time-of-day input, two-person age difference, historical local-calendar reconstruction or medical assessment.