Calcylator
Age & dates

Age calculations,
explained

Years, months and days: how age is worked out, and why the exact answer depends on the calendar.

Calcylator Editorial Team

Updated · 4 min read

Working out exact age

Age is the time between a date of birth and a chosen date, usually today. The quick method subtracts the birth year from the current year. That can be a year out, because it ignores whether the birthday has happened yet.

The exact method subtracts days, months and years separately and borrows when a part comes out negative:

  1. Subtract the days. If the answer is negative, take one month off the month count and add the number of days in the month before the end date.
  2. Subtract the months. If the answer is negative, take one year off the year count and add 12 months.
  3. Subtract the years.

  • Date of birth

    28 Nov 1995

  • Age on

    15 Mar 2026

Exact age

30 years, 3 months, 15 days

Days: 15 − 28 is negative, so borrow February 2026’s 28 days: 15 − 28 + 28 = 15, and one month is used. Months: (3 − 1) − 11 is negative, so borrow a year: 12 + 2 − 11 = 3. Years: 2026 − 1995 − 1 = 30.

Check the answer by adding it back to the birth date: 28 Nov 1995 plus 30 years is 28 Nov 2025, plus 3 months is 28 Feb 2026, plus 15 days is 15 Mar 2026. If you land on the end date, the sum is right.

Borrowing from the month before the end date is the most common convention. Some tools borrow from the birth month instead, which can shift the day count by a day or two, so small differences between calculators are normal.

Age in days and weeks

Age in days is a straight count of the days between two dates, leap days included. From 28 Nov 1995 to 15 Mar 2026 that is 11,065 days, or 1,580 weeks and 5 days.

Months run from 28 to 31 days, so you can’t get this by multiplying years by 365. It is the one place where a calculator beats mental maths.

Decide whether the end date counts. A plain subtraction counts the days that pass between the two dates; if you want both the first and last day included, add 1. Milestones are a popular use: someone born on 28 Nov 1995 reached 10,000 days on 15 Apr 2023.

Hours follow from days: 11,065 × 24 = 2,65,560 hours. Treat that as approximate unless you also know the time of birth.

Age on a future or past date

The same method works for any end date, which helps with eligibility checks, admissions and retirement planning. Replace “today” with the date you care about.

  • Date of birth

    28 Nov 1995

  • Age on

    1 Jan 2030

Exact age

34 years, 1 month, 4 days

That is 12,453 days in total.

Insurers and some forms ask for “age last birthday”, which is your completed years, or “age nearest birthday”, which rounds to the closer birthday. The two can differ by a year, so read the wording before you fill in a form.

Leap years and 29 February

A leap year adds 29 February. Every year divisible by 4 is a leap year, except century years, which must also be divisible by 400. So 2000 was a leap year and 1900 was not. A year such as 2100 divides by 4 and by 100 but not by 400, so it will not be a leap year either.

Leap days are why 365 × years always comes up short. From 28 Nov 1995 to 28 Nov 2025 there are 10,958 days, not 10,950 (30 × 365), because 8 leap days (1996 to 2024) fall in between.

A person born on 29 February has a calendar birthday only every fourth year. In other years they usually mark it on 28 February or 1 March, and the legal rule differs between countries.

How some cultures count age

Not everyone counts age the way most calculators do. In traditional East Asian age reckoning, a baby is one year old at birth and gains a year at the new year, not on the birthday. That makes a person one or two years older than their international age. South Korea moved official and legal use to the international method in 2023.

The method in this article, and in most age calculators, counts completed years since birth.

Common questions

How do I calculate my exact age in years, months and days?

Subtract your birth date from the target date part by part: days, then months, then years. If days come out negative, borrow a month and add the days in the previous month. If months come out negative, borrow a year and add 12. An age calculator does this instantly.

How many days old am I?

Count the days between your date of birth and today, leap days included. Multiplying years by 365 undercounts. For example, someone born on 28 November 1995 was 11,065 days old on 15 March 2026. A days-between-dates calculator gives the exact figure.

How do I find my age on a future date?

Use the same subtraction, but replace today’s date with the future date. Someone born on 28 November 1995 will be 34 years, 1 month and 4 days old on 1 January 2030. This is handy for checking eligibility cut-offs.

How does a leap year affect age?

A leap year adds one day, so the day count between two dates is higher when the period spans 29 February. Your age in completed years is unaffected, except for people born on 29 February, who usually celebrate on 28 February or 1 March in non-leap years.

Is age counted differently in different countries?

Most countries count completed years since birth. Traditional East Asian systems count a newborn as one year old and add a year at the new year. South Korea switched official use to the international method in 2023, which is the method used here.

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