It’s half past eleven at night. You’re filling in a form — a job application, a passport renewal, your child’s school admission — and it doesn’t just ask for your age. It wants years, months and days. You look at your date of birth, try to subtract it from today’s date in your head, and somewhere around the months it all falls apart.
There is a simple paper-and-pen method for this. It’s called borrow-and-carry, it’s the same idea you learned for subtraction in school, and once you’ve done it twice you’ll never get stuck on one of those forms again.
Why you can’t just subtract like normal numbers
Subtracting years is easy: 2026 minus 1991 is 35. Done.
But dates aren’t normal numbers, because months aren’t all the same size. You can’t treat “October minus March” the way you’d treat “10 minus 3” and call it a day — well, actually you can for the months, but the days column needs special handling. If your birthday is on the 28th and today is the 15th, you can’t subtract 28 from 15. You have to “borrow” days from the previous month, exactly like borrowing in long subtraction.
That’s the whole trick. Three columns — days, months, years — worked right to left, borrowing when the top number is too small.
The method, step by step
Grab a pen. Write today’s date on top and your birth date underneath, in day–month–year order.
Step 1 — Days. Subtract the birth day from today’s day. If today’s day is smaller than your birth day, borrow the full length of the previous month: add that many days to today’s day, do the subtraction, and remember that the current month has now “lost” one — you’ll use one less month in the next step.
Step 2 — Months. Subtract the birth month from the current month (using the reduced month if you borrowed). If the current month is smaller, borrow 12 months: add 12, subtract, and reduce the year by one for the next step.
Step 3 — Years. Subtract. Whatever’s left is your age in years.
Let’s do it for real, twice — once clean, once with borrowing.
Worked example 1: no borrowing needed
Say you were born on 14 March 1991, and today is 15 October 2026.
- Days: 15 − 14 = 1 day
- Months: October is the 10th month, March the 3rd. 10 − 3 = 7 months
- Years: 2026 − 1991 = 35 years
Your exact age: 35 years, 7 months and 1 day.
Nothing borrowed, nothing complicated. Most of the time it looks exactly like this.
Worked example 2: borrowing in both columns
Now the interesting one. Born 28 November 1998, today 15 October 2026.
Days first: 15 is smaller than 28, so we borrow. The previous month is September, which has 30 days. Add them: 15 + 30 = 45. Now subtract: 45 − 28 = 17 days. And because we borrowed, October (month 10) counts as September (month 9) in the next step.
Months next: 9 is smaller than 11 (November), so borrow 12 months: 9 + 12 = 21. Subtract: 21 − 11 = 10 months. The year drops by one: 2026 becomes 2025.
Years last: 2025 − 1998 = 27 years.
Your exact age: 27 years, 10 months and 17 days.
Quick sanity check: 28 November 1998 plus 27 years is 28 November 2025. Add 10 months → 28 September 2026. Add 17 days → 15 October 2026. It lands exactly on today. The method works.
The two mistakes almost everyone makes
Borrowing the wrong number of days. When you borrow for the days column, you borrow the length of the previous month — the month before today’s month, not your birth month. January has 31, April 30, and February needs a glance at the year: 29 days in a leap year, 28 otherwise. (A year is a leap year if it divides evenly by 4 — with the fussy exception of century years, but for borrowing purposes, just check a calendar if you’re unsure.)
Counting your birthday as day one. On your birthday, your age in months and days is zero — you are exactly 35, not 35 years and 1 day. Age starts counting from the birthday, not including it.
In a hurry? The 30-second version
For casual conversation you don’t need all this. Take the current year minus your birth year, then subtract one if your birthday hasn’t happened yet this year. Born in 1991, it’s October 2026, birthday was in March → 2026 − 1991 = 35. Done. It won’t satisfy a form that asks for months and days, but it’s right for every normal “how old are you” question.
Frequently asked questions
Why does an online age calculator sometimes disagree with my hand calculation by a day?
Usually it’s a time-zone or rounding difference — some calculators count from midnight in a different zone, or they include/exclude the current day differently. A one-day wobble between tools is normal and nothing to worry about; for official forms, use the borrow-and-carry result.
Do I count the day I was born as day one of my age?
No. The day you’re born, you’re zero days old — age starts at zero and builds from there. That’s why on your birthday the months-and-days part of your exact age resets to zero.
I was born on 29 February. How does this work in non-leap years?
Your age still counts in full years — you turn a year older on 28 February or 1 March depending on local convention (most places use 1 March legally, 28 February socially; it varies). The borrow-and-carry method handles it fine either way: just borrow 28 days for February in non-leap years.
How should I write my exact age on official forms?
Write it exactly as the form asks — usually “35 years, 7 months, 1 day” or in three separate boxes. Don’t round, and don’t convert the months into a decimal. If the form only asks for years, give completed years (35, not “almost 36”).
Can’t I just subtract the birth year from the current year?
For your age in years, yes — with the birthday adjustment above. But that tells you nothing about the months and days, which is what forms, visa applications and pension calculations usually want. That’s when you need the full method.
How this guide was made
This guide was written by the Armtema team and edited for plain-English clarity. The borrow-and-carry method was checked by working both examples through twice against a calendar, including the September 30-day borrow and the leap-year February rule. The sample dates are made-up round examples so you can follow the arithmetic. Last reviewed: October 2026.