Date of birth from a known age.
What it's for
Work backwards from an age to a date of birth — for example when a document says someone was "35 years, 2 months and 10 days old" on a certain day.
Before you start
Have the age, as exactly as you know it, and the date the person was that age — today, or the day a form was filled in or a document signed.
Say how much of the age you know
Pick I know years, months and days if you have the full age, or I only know the years if all you have is something like "35 years old".
Enter the age
Type the years in Years old. With the full age, also type Months and Days — use 0 for none.
Choose the date of that age
Aged that on starts as today. Change it to the day the age was true, such as the date on the document. The answer depends entirely on this date.
Press Calculate
Press Calculate. The answer already updates as you type; the button brings it into view. Under Calculation is how the date was counted back.
Reading the result
If it goes wrong
The Age Calculator run backwards: from an age on a known date, to the birthday that produces it. The two share the same calendar code, so they always agree.
The part most tools get wrong is that the inverse is not unique. Because months differ in length, several birthdays can give the same age in years, months and days. This one subtracts, then checks its answer by running the age forward again, and reports every birthday that matches rather than quietly choosing one.
Because more than one birthday can produce the same age, and most calculators hide that. Months are different lengths, so an age that lands near a month end does not fix a single day. Rather than pick one of several right answers and present it as the answer, the tool lists them all and shows the earliest.
Then switch to the years-only mode, which gives a window rather than a date. "She was 35 when the deed was signed" narrows a birthday to a span of about a year — anything else is a guess dressed as a fact.
It is the same arithmetic run backwards, and the two share the calendar code that does it. A test asserts they agree across hundreds of ages, so a birthday from this tool always produces the age you entered when fed back into the other.
It is the date the age was true on. Today by default, but you can set it to any date — the day a form was filled in, or a document signed. The birthday you get depends entirely on it.
Subtract the months, then the days. Then run the age forward from the candidate and see whether it lands on the age you asked for — and try a few days either side, because it may not be the only one that does.
One month from 31 January is 28 February, because there is no 31st. One month from 30 January is also 28 February for the same reason. Two different starting days, the same answer — so running that backwards cannot recover which one you meant.
A tool that hands you a single date from an age in whole years has invented the other 364 days of precision.
Important
Calendar arithmetic only. Where several birthdays give the same age, all of them are shown rather than one being picked for you. Nothing you type here is sent to our servers — the calculation runs entirely in your browser.
