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Age and birthday calculator

Find an exact calendar age, total elapsed days, the weekday of birth and when the next birthday falls.

Age from calendar dates

Choose the date on which the age should be measured.

Choose a date of birth and an age-on date. Use Today for a current-age calculation that remains reproducible in the URL.

Included

  • Exact calendar age in completed years, months and remaining days
  • Total elapsed calendar days and full weeks plus remaining days
  • The weekday on which the person was born
  • The next birthday date, weekday, countdown and age reached
  • A choice of 28 February or 1 March for leap-day birthdays in non-leap years

Not included

  • Clock times, birth times, time zones and partial days
  • Legal age, school-year, pension, benefit or eligibility rules
  • Lunar, religious or non-Gregorian calendars
  • Life-expectancy, health or astrological interpretations

What this means

Calendar age is not just elapsed days divided by 365. The calculator first counts completed birthday anniversaries, then completed calendar months since the latest anniversary, and finally the remaining calendar days.

Total days use the direct date difference Age-on date − Date of birth. Full weeks and remaining days are another exact way to split that same elapsed-day total; they do not approximate years or months.

A birthday on 29 February needs an explicit convention in a non-leap year. Choose 28 February or 1 March. That choice controls when the completed year changes and which date is shown as the next birthday; it is a calculation convention, not legal advice.

Formula & worked example

total days = Age-on date − Date of birth
completed years = whole birthday anniversaries reached
completed months = whole calendar months after the latest birthday
remaining days = days after the latest completed month
days until next birthday = Next birthday − Age-on date

Born 15 May 1990; calculate age on 20 July 2026

Completed birthday anniversaries
36 years
From 15 May to 15 July
2 completed months
From 15 July to 20 July
5 remaining days

The exact calendar age is 36 years, 2 months and 5 days. The next birthday is 15 May 2027, when the person turns 37.

How this calculation works

The calculation follows the formulas, definitions and assumptions explained on this page. The references below support the method and any stated boundaries.

Official sources

  • ISO — ISO 8601 date and time format — Unambiguous Gregorian calendar dates written in year-month-day order; the age decomposition and leap-day anniversary rule are stated calculator conventions

Common questions

Why do I need to choose an age-on date?
It makes the result reproducible. A shared link gives the same answer later and on another device instead of silently changing every midnight. Use the Today shortcut when you want the current local date.
How are months handled after a date such as the 31st?
A completed month lands on the same day number where possible. If that month is shorter, it lands on the month's final day. Remaining days are counted from that clamped date.
Which leap-day option is legally correct?
This calculator does not decide legal birthdays or age thresholds. Laws and institutions can use different rules. The selected option is an explicit calendar convention for this result only.
Does the day of birth count as one day old?
No. On the birth date, elapsed age is zero years, zero months and zero days. One full day has elapsed on the following calendar date.

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