How it is worked out
date of birth = reference date − age
Why it is done this way
Whole years are not enough for a date: a 30-year-old may have been born 30 years ago yesterday or almost 31 years ago. What comes out is a one-year range, and it only narrows to a single date if you add the completed months and days.
Step by step
- Subtract the whole years from the reference date: that is the youngest end
- Subtract one more year and add a day: that is the oldest end
- Between the two there are 365 days, or 366 when the range catches a leap day
- If you know the completed months and days, subtract those too and the date is unique
What is worth knowing
This calculation comes up when filling in forms that only record an age, and in genealogy, where a headstone or a census gives the years but not the date. Bear in mind that an old census records the age as declared, which was rounded down and often to a comfortable number: nineteenth-century registers show a striking pile-up of ages ending in zero and five.