Know an age but not the birthday? Enter the age in years (plus months and days if known) and the date it applies to, and get the date of birth with its weekday — or the full range of possible birth dates when only the years are known.
How the date of birth calculator works
Enter an age and the date it applies to. The calculator subtracts the years and months calendar-style, then the days exactly, and returns the date of birth with its weekday. If you know only the age in completed years, it also shows the full window of possible birth dates — everyone born between those two dates is that age on the chosen day. This is the reverse of our age calculator, so you can check a result both ways.
When it is useful
- Filling a date of birth on a form when you know someone’s age at a known date
- Checking a stated age against a document date
- Working out the birth year from an age on a school certificate, a resume or a casting sheet
- Finding the earliest and latest birthdays that match “aged 25” today
Frequently asked questions
Can I find a date of birth from age alone?
With years only, you get a range of possible birthdays (about a year wide). Add months and days to get an exact date.
What date does the age apply to?
By default today. Change “On this date” to the date on which the age was stated — a document date, an exam date or an event date.
How accurate is the result?
Exact when years, months and days are all given. Year-and-month ages can shift a day at month ends because months differ in length; the note under the result says when that may apply.
What does “days lived” mean?
The exact number of days between the calculated birth date and the chosen date, including leap days.
Is anything stored?
No. The calculation runs in your browser and nothing you enter is sent to ZJELL Models.
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