Most of us can say how old we are without thinking, because we only track whole years. The moment you need the exact figure, for a school form, a medical chart, a visa application or simply a round-number celebration, the arithmetic turns out to be fiddlier than it looks. Months have different lengths, February changes every four years, and the answer depends on whether you count the start and end days. This guide walks through the method people have used on paper for generations, shows exactly where the traps are, and explains why two age calculators can sometimes give answers a day apart.
Why exact age is trickier than it looks
A year is a clean unit: you gain one on each birthday, so subtracting your birth year from the current year and taking one off if your birthday has not arrived yet always gives the right answer. Months and days are messier because they are not the same size. March has 31 days, April has 30, and February has 28 or 29. If you were born on 31 January, there is no 31 February for your “one month old” day to land on, so any method has to decide what happens at the end of a short month.
The convention almost everyone uses is to count whole calendar months first and then count the leftover days. You are one month older each time the day of the month you were born on comes round again, and when that day does not exist in a given month, the month is completed on its last day. After the months are counted, the remaining days are simply counted on the calendar. That is the logic behind the borrowing method below, and it matches what most published age calculators and official forms expect.
The borrowing method, step by step
The method is ordinary column subtraction, the kind you learned for large numbers at school, with one twist: when you borrow from the months column you borrow the length of a real month rather than ten. Write today’s date above your date of birth, both in year, month, day order, and work from right to left.
- Subtract the days. If today’s day is equal to or larger than your birth day, subtract and move on. If it is smaller, borrow one month: reduce today’s month by one and add the number of days in the previous calendar month to today’s day, then subtract.
- Subtract the months. If today’s month (after any borrowing) is equal to or larger than your birth month, subtract. If it is smaller, borrow one year: reduce today’s year by one and add 12 to the month, then subtract.
- Subtract the years. Take your birth year from today’s year, after any borrowing. The three results are your age in years, months and days.
The one decision that trips people up is which month to borrow. The rule is to borrow the month that ends just before today’s month. If today is 10 March, the month you have most recently completed is February, so you borrow 28 days, or 29 in a leap year. If today is 10 May, you borrow April’s 30 days. Borrowing the wrong month is the single most common source of errors in hand calculations, and it can make the answer one, two or even three days out.
Two worked examples
Start with an easy one, where nothing needs to be borrowed. Someone born on 12 April 1990 wants to know their exact age on 30 October 2026. The days column gives 30 minus 12, which is 18. The months column gives 10 minus 4, which is 6. The years column gives 2026 minus 1990, which is 36. So on that date they are 36 years, 6 months and 18 days old. Counted in days, the same span is 13,350 days, because those 36 years contain nine leap days (1992, 1996 and every fourth year to 2024).
The second example needs borrowing in both columns, and it is the one shown in the diagram at the top. Someone born on 25 August 1987 wants their age on 10 March 2026. In the days column, 10 is smaller than 25, so we borrow a month. Today’s month drops from 3 to 2, and because the month before March is February and 2026 is not a leap year, we add 28 days to get 38. Now 38 minus 25 is 13. In the months column, 2 is smaller than 8, so we borrow a year: the year drops from 2026 to 2025 and the month becomes 14. Now 14 minus 8 is 6. Finally, 2025 minus 1987 is 38. The exact age is 38 years, 6 months and 13 days.
Quick check. Count forward instead of back. From 25 August 1987, 38 years takes you to 25 August 2025, six more months to 25 February 2026, and from 25 February to 10 March 2026 is 13 days (three days to the end of February, then ten in March). The forward count and the subtraction agree, which is a good habit whenever the answer matters.
Birthdays on 29 February
About one person in every 1,461 is born on a leap day, and their age raises a fair question: when does the birthday happen in the three years out of four that have no 29 February? For counting age in years, nothing unusual happens. A person born on 29 February 2000 was 26 years old throughout most of 2026, exactly like everyone else born in 2000. Only the exact moment of the birthday is in doubt.
For everyday celebrations, most leap-day babies pick either 28 February, so the birthday stays in the same month, or 1 March, because it is the day after 28 February just as 29 February would have been. For legal purposes, such as the age at which someone can drive, vote or sign a contract, the answer depends on local law and is not the same everywhere, so it is worth checking the rules in your own country rather than relying on a general answer. For hand calculations, the safest approach is the forward-count check above, which makes it obvious which day the months and years are completed on.
Age in months, weeks and days
Sometimes the useful number is not years, months and days, but a single unit. Babies are usually described in weeks and then months, and many milestones, such as the 10,000-day birthday, are counted in days. Converting from the years-months-days answer is straightforward for months: multiply the years by 12 and add the months. The person in our first example, 36 years and 6 months, is 438 completed months old.
Days and weeks are better counted directly than converted, because a year is not a whole number of days. The direct count is the number of calendar days between the two dates, which automatically includes every 29 February in between. Divide that by seven and drop the remainder for completed weeks. For the first example, 13,350 days is 1,907 full weeks with one day left over. Multiplying 36 years by 365 would have given 13,140 and then you would still have to add the 201 days since the last birthday and remember the nine leap days, which is exactly why a direct count is safer.
| Unit | Best way to get it | Example (12 Apr 1990 to 30 Oct 2026) |
|---|---|---|
| Years, months, days | Borrowing method | 36 y, 6 m, 18 d |
| Months | Years × 12 + months | 438 months |
| Days | Count calendar days | 13,350 days |
| Weeks | Days ÷ 7, drop remainder | 1,907 weeks |
Doing it in a spreadsheet
Excel and Google Sheets both include a function called DATEDIF that does most of this for you. With the date of birth in cell A1 and today’s date in B1, the formula =DATEDIF(A1,B1,"Y") returns completed years, =DATEDIF(A1,B1,"YM") returns the months left over after the years, and =DATEDIF(A1,B1,"M") returns total completed months. Subtracting one date from the other, =B1-A1, gives the number of days directly, because spreadsheets store dates as day numbers.
The weak spot is the leftover days. DATEDIF has an “MD” option meant to return the days after the months, but Microsoft’s own documentation warns that it can give wrong results, including negative numbers, around the ends of months. A safer formula for the days part is to subtract the date of your last “month birthday” from today, for example =B1-EDATE(A1,DATEDIF(A1,B1,"M")), which uses EDATE to step forward the exact number of completed months.
Why calculators disagree
If you check your age on two websites and get answers a day apart, neither is necessarily broken. There are three common reasons for the difference. The first is whether the count includes both ends. Some tools count the day of birth and today as two extra days, which is the convention for things like hotel nights or project durations but not for age. The second is time zone: a calculator that runs on a server may use that server’s date, which can already be tomorrow or still be yesterday where you are. Ours runs in your browser, so it uses your own date. The third is the month-end rule. A few tools treat 31 January plus one month as 3 March rather than 28 February, which changes the days figure without changing the total day count.
The total number of days between two dates is the one figure that never depends on these choices, as long as both ends are handled the same way. That is why it is a good number to keep in mind when you check anything else, and it is why our days alive calculator leads with it.
Other ways of counting age
Everything above uses the international convention, where you are zero at birth and gain a year on each birthday. It is now the official system almost everywhere. South Korea was one of the last countries where a different count was common in daily life: under “Korean age”, a baby was one year old at birth and everyone gained a year on 1 January. Since 28 June 2023 the law has used international age for official purposes, although the traditional count still appears in conversation. Some other East Asian traditions once used similar reckonings, and many cultures mark specific ages, such as the first birthday or the sixtieth, with special ceremonies, but the arithmetic for the age itself is the same.
Whichever way you count, the borrowing method gives an answer you can trust, as long as you borrow the month that ends just before today’s month and check the result by counting forward. When you want the number without the arithmetic, or want to see it in months, weeks, days and even seconds at once, our calculators do it all in your own time zone.
Questions people ask
Do I count the day I was born?
No. Age counts completed time, so on the day you are born you are zero days old, and you turn one day old at the end of that first day. The same logic makes you one year old on your first birthday, not the day before.
How many days do I borrow when the days do not subtract?
Borrow the number of days in the month before the current month. If today is in March, you borrow February’s 28 days (29 in a leap year). If today is in May, you borrow April’s 30.
What is chronological age?
It is simply your age measured from your date of birth, usually written as years, months and days. Schools, doctors and test publishers use the term to separate it from things like reading age or bone age.
Does Korea still use a different age system?
Not for official purposes. Since 28 June 2023 South Korean law uses international age, counted from birth with zero at birth. The older “Korean age”, where everyone turned a year older on 1 January, still turns up in casual conversation.
How old is someone born on 29 February?
Exactly as old as anyone else born that year. They have fewer actual birthdays, about one every four years, but their age in years grows every year like everyone else’s.
Sources. Gregorian calendar rules; Microsoft Support documentation for the DATEDIF function; Government of the Republic of Korea, adoption of international age in law (in force 28 June 2023).