Chronological age is calculated by subtracting a date of birth from a reference date (usually today’s date), working right to left through days, months, and years. The process is straightforward when the numbers line up neatly, but it requires a borrowing technique similar to basic subtraction when the current day or month is smaller than the birth day or month. Here’s exactly how to do it.
The Basic Subtraction Method
Write the two dates in a vertical column format, with the reference date (today) on top and the date of birth on the bottom. Arrange them in three columns: year, month, day. Then subtract right to left, starting with days, then months, then years.
For example, if today is June 15, 2025 and the date of birth is March 10, 2018:
- Days: 15 − 10 = 5
- Months: 6 − 3 = 3
- Years: 2025 − 2018 = 7
The chronological age is 7 years, 3 months, 5 days.
How Borrowing Works
When the current day is smaller than the birth day, you can’t subtract directly. You need to borrow from the months column, and the standard convention is to add 30 days to the day column while subtracting one month. If the months column also comes up short after borrowing, you borrow from the years column by converting one year into 12 months.
Here’s an example that requires borrowing in both columns. Say today is February 1, 2025 and the date of birth is November 30, 2017:
Start by writing it out:
- Reference date: 2025 / 2 / 1
- Date of birth: 2017 / 11 / 30
You can’t subtract 30 from 1, so borrow 30 days from the month column. The day column becomes 1 + 30 = 31, and the month column drops from 2 to 1. Now subtract days: 31 − 30 = 1.
Next, you can’t subtract 11 from 1 in the months column, so borrow 12 months from the year column. The month column becomes 1 + 12 = 13, and the year column drops from 2025 to 2024. Subtract months: 13 − 11 = 2. Subtract years: 2024 − 2017 = 7.
The chronological age is 7 years, 2 months, 1 day.
Rounding to Years and Months
Many standardized tests and developmental assessments express age in just years and months, without the days. The standard rounding rule: if the remaining days are 15 or more, round up to the next month. If the days are 14 or fewer, drop them and keep the last full month.
A child whose calculated age is 12 years, 7 months, 25 days would be recorded as 12 years, 8 months. A child at 6 years, 10 months, 11 days would be recorded as 6 years, 10 months. This rounding convention is widely used in educational and psychological testing when looking up scores on norm tables.
Why 30 Days, Not 31?
You might wonder why borrowing always uses 30 days instead of the actual number of days in the previous month. The answer is standardization. Months vary from 28 to 31 days, and using a flat 30 keeps the calculation consistent regardless of which month you’re working with. This is the convention used by most clinical, educational, and developmental assessment guidelines. It introduces a tiny margin of imprecision (a day or two at most), but it makes the math uniform and reproducible across different practitioners.
Calculating Age in Total Days
Sometimes you need age expressed as a single number of days rather than years, months, and days. The simplest manual approach is to multiply the number of complete years by 365, then count the remaining days from your last birthday to today using the actual number of days in each month.
You’ll also need to account for leap years. A leap year occurs every four years (2020, 2024, 2028), except for century years not divisible by 400. Count how many February 29ths fell between your birth date and today, then add that number to your total. For someone who has lived through 10 leap years, that’s 10 extra days.
This method gets you within a day or two of the exact figure. For a perfectly precise count, you can convert both dates to Julian Day Numbers (a continuous count of days used in astronomy) and subtract one from the other, but for most manual purposes, the multiply-and-add approach works well.
Converting to Decimal Years
Some formulas and growth charts use age as a decimal rather than years and months. To convert, take the months portion and divide by 12, then add it to the years. A child who is 7 years and 3 months old has a decimal age of 7 + (3 ÷ 12) = 7.25 years. If days are involved, divide the days by 365 and add that too. An age of 15 years, 4 months, 28 days works out to roughly 15 + (4 ÷ 12) + (28 ÷ 365) = approximately 15.41 years.
Leap Day Birthdays
If you were born on February 29, calculating your age in non-leap years raises an obvious question: when does your birthday fall? Legal scholars generally treat March 1 as the effective birthday in non-leap years. The reasoning is that February 29 is the day after February 28, and in a non-leap year, the day after February 28 is March 1. So a person born on February 29 is considered to have aged one year on March 1 when there’s no leap day.
For manual age calculations, this means you’d use March 1 as the reference birthday in non-leap years when precision matters. In leap years, use February 29 as usual.
Adjusted Age for Premature Infants
For babies born before 37 weeks of gestation, health providers often use a corrected (or gestation-adjusted) age rather than straight chronological age. The calculation adds one extra step: subtract the number of weeks of prematurity from the chronological age.
To find weeks of prematurity, subtract the baby’s gestational age at birth from 40 weeks (the standard term). A baby born at 32 weeks was 8 weeks premature. If that baby’s chronological age is 20 weeks, the corrected age is 20 − 8 = 12 weeks. This adjusted age is typically used for plotting growth charts and evaluating developmental milestones until the child reaches age 2, or age 3 for infants who weighed less than about 2.2 pounds at birth. Once a child’s growth catches up to their peers, chronological age replaces the corrected age.
A Quick Practice Problem
Try this one yourself. The reference date is January 5, 2025. The date of birth is September 22, 2019.
Set it up: 2025 / 1 / 5 minus 2019 / 9 / 22.
Days: 5 is less than 22, so borrow 30 days. Days become 35, months drop from 1 to 0. Subtract: 35 − 22 = 13 days.
Months: 0 is less than 9, so borrow 12 months from the year. Months become 12, years drop from 2025 to 2024. Subtract: 12 − 9 = 3 months.
Years: 2024 − 2019 = 5 years.
The chronological age is 5 years, 3 months, 13 days. Using the rounding rule, since 13 is less than 15, this rounds to 5 years, 3 months for assessment purposes.