Introduction
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A parent we know once spent an entire evening on this. Her daughter’s school form said the child must be “5 years old as on 31 March” — not roughly five, not turning five soon. Five, on that exact date, in years, months and days. She did the sum three times and got three different answers.
If that sounds familiar, you are in good company. The years are never the problem. The problem is the scraps left over: a few months here, some days there, and months that refuse to all be the same length. Below is the paper method teachers themselves use. It is not fast, flashy maths — it is careful column work, and it lands on the right answer every time, including the awkward February cases.
Before You Start: What “Exact Age” Means
Exact age is three parts: completed years (birthdays fully passed), completed months since the last birthday, and the remaining days after that. Two dates are involved, and mixing them up causes almost every error:
- Date of birth (DOB) — the starting date.
- Reference date — the date on which you want the age: today, or a deadline like “age as on 1 September”.
Write both in the same order — day, month, year — before you begin.
The Column Method: Step by Step
Write the reference date on top, the date of birth underneath, in three columns: Day | Month | Year. Then subtract in this order — days first, then months, then years — borrowing as needed.
The borrowing rules
- Days column: If the top day is smaller than the birth day, borrow one month from the reference month. The borrowed month is worth the number of days in the month before the reference month (the month you are counting back through). Then reduce the reference month by 1.
- Months column: If the top month (after any borrowing above) is smaller than the birth month, borrow one year: add 12 to the month, and reduce the reference year by 1.
- Years column: Subtract normally once the other columns are settled.
Read that days rule once more, slowly. Honestly, it is the single place where most people go wrong — including, the first time, us. Borrow in March and you are borrowing February’s days: 28 of them, or 29 in a leap year. Not March’s 31. Get this one detail right and the rest of the method almost does itself.
First, an easy one
No borrowing, no tricks — just to see the columns do their work.
Date of birth: 14 August 2001
Reference date: 20 October 2026
Step 1: Days
20 − 14 = 6 days. No borrowing needed; the reference day is larger.
Step 2: Months
10 − 8 = 2 months. No borrowing needed.
Step 3: Years
2026 − 2001 = 25 years.
Exact age: 25 years, 2 months, 6 days.
Now the one that will actually resemble your form — a December baby, a March deadline, and both borrowings in a single sum:
Date of birth: 25 December 1998
Reference date: 10 March 2026
Step 1: Days — borrowing needed
10 is smaller than 25, so borrow one month from March. The month before March is February — 28 days in 2026, not a leap year.
Days: 10 + 28 = 38, and 38 − 25 = 13 days. The reference month drops from 3 to 2.
Step 2: Months — borrowing needed
Now compare 2 (March, reduced) with 12 (December): 2 is smaller, so borrow one year: 2 + 12 = 14.
14 − 12 = 2 months.
The reference year drops from 2026 to 2025.
Step 3: Years
2025 − 1998 = 27 years.
Exact age: 27 years, 2 months, 13 days.
Check it forwards: 25 December 1998 + 27 years = 25 December 2025; + 2 months = 25 February 2026; + 13 days = 10 March 2026. It lands exactly on the reference date, so the answer is right. This twenty-second check catches nearly every slip.
Leap Years and February Birthdays
A leap year comes every 4 years (years divisible by 4), with a century exception: years divisible by 100 are not leap years unless also divisible by 400. So 2000 was one, 1900 was not, 2024 was, 2026 is not.
Leap years matter twice here. When you borrow through February, it has 29 days in a leap year and 28 otherwise — check the reference year, not the birth year. And people born on 29 February only see that date every four years; in other years, forms treat their birthday as 28 February or 1 March, depending on the organisation’s rule. Follow that rule.
Common Mistakes to Avoid
- Counting the unfinished year. Age means completed years: born in November 2000, you are still 25 in October 2026.
- Borrowing 30 days for every month. The borrowed value is the real length of that specific previous month.
- Forgetting to reduce the next column after borrowing. A borrow always takes from somewhere.
- Using today’s date instead of the “as on” date. The two can differ by a whole year.
- Mixing date formats. 05/06 can mean 5 June or May 6 — write month names out.
Frequently Asked Questions
“Can’t I just do years times 365 to get my age in days?”
You can, for a rough guess — but it will be short. Leap years slip an extra day in roughly every four years, and your current, unfinished year adds a chunk on top. For a better total: completed years × 365, plus one day for each leap year you have lived through, plus the days since your last birthday.
“My baby was born this morning. Is she 0 days old or 1 day old?”
On paper, 0 — forms count only completed units, which is why a newborn’s age reads 0 years, 0 months, 0 days even while she is very much here. By tomorrow morning she will be 1 day old in the way most hospitals count it.
“The form only has one box: age in years. What do I write?”
Your completed years — the years figure from the full calculation. Quick sanity check: if your birthday hasn’t happened yet this year, the number is one less than the plain year subtraction suggests. That one catches a surprising number of adults out, not just parents doing school forms.
Conclusion
Write both dates down. Borrow February’s real length, not a tidy 30. Then add your answer forwards and watch it land on the deadline date. Three habits, and that school form stops being an evening’s worry and becomes five minutes with a pencil.