How age is counted
Age is counted the way birthdays work. You complete a year on each birthday and a month on the same day of each following month. Whatever is left after the last complete month is a count of days. Someone born on 20 June is 2 months old on 20 August, and 2 months and 26 days old on 15 September.
The totals are exact counts of calendar days, so they include every leap day. Age in weeks is the total days split into weeks of seven; age in months is the complete months plus the leftover days.
Because months have different lengths, a date near the end of a month needs a rule. When the birthday’s day doesn’t exist in a month — the 31st in a 30-day month, or 29–31 in February — the calculator uses that month’s last day. A baby born on 31 January is one month old on 28 February (29 February in a leap year).
For the gap between any two dates — not just from a birthday — use the days between dates calculator. To find the date a number of days, weeks or months away, use the date calculator, and for hours and minutes the time calculator.
The method
First count whole months, using only the year and month numbers:
months = (Y₂ − Y₁) × 12 + (M₂ − M₁)
If the birthday’s day of the month hasn’t been reached yet in the final month, take one away. Then split the months into years and months, and count the remaining days:
years = months ÷ 12 (whole part) · days = date − (birth date + months)
- Y₁, M₁
- year and month of birth
- Y₂, M₂
- year and month of the date you are measuring on
Total days come from subtracting serial day numbers — a count of days from a fixed starting point — so leap years are handled exactly. The calculator converts dates with Howard Hinnant’s days-from-civil algorithm on the Gregorian calendar, where a year is a leap year if it divides by 4, except century years that don’t divide by 400 (2000 was a leap year, 1900 was not).
Worked example
Someone born on Wednesday, 20 June 1990, on 15 September 2026:
- Whole months by the calendar: (2026 − 1990) × 12 + (9 − 6) = 435. The 20th of September hasn’t arrived, so take one away: 434 months = 36 years, 2 months.
- 20 June 1990 + 434 months = 20 August 2026. From there to 15 September is 11 + 15 = 26 days.
- Age: 36 years, 2 months, 26 days.
- Total days: 36 × 365 + 9 leap days (1992 to 2024) = 13,149 days to 20 June 2026, plus 87 days to 15 September = 13,236 days, which is 1,890 weeks and 6 days, or about 317,664 hours.
- The next birthday, the 37th, falls on Sunday, 20 June 2027 — 278 days away.
Birthdays on 29 February
People born on 29 February have a real birthday only in leap years. In the other three years, this calculator counts the birthday on 28 February, the last day of the birth month. On that day the age goes up by a year and the days count returns to zero.
The legal answer depends on where you are. Scots law, for example, says the anniversary in a common year is 1 March (Age of Legal Capacity (Scotland) Act 1991, section 6). If a legal age — for a licence, a contract or a benefit — depends on it, check the rule that applies to you. Either way, the difference is one day, and only in common years.
Things the calculator doesn’t know
- Time of birth. Ages are counted in whole days. Hours are simply days × 24, so they can be off by up to a day’s worth, and daylight-saving changes aren’t counted.
- Time zones. Each date is taken as the calendar date where it happened. Someone born late in the evening in New York was born on the next day by the calendar in London.
- Other age systems. Some cultures have counted age differently, for example counting a newborn as one year old. This calculator gives the international, completed-years age.
- Historic dates. Dates are on the Gregorian calendar throughout. Records from countries that still used the Julian calendar — Britain and its colonies before 1752, Russia before 1918 — are 10 to 13 days behind the Gregorian date.
Frequently asked questions
How do I calculate my exact age?
Count the complete years to your last birthday, then the complete months since then, then the days since that month’s date. For example, from 20 June 1990 to 15 September 2026 is 36 years to 20 June 2026, 2 months to 20 August, and 26 more days.
How old is someone born on 29 February in a common year?
This calculator treats 28 February as the birthday in years without a 29 February, so the age goes up on 28 February. Some laws use 1 March instead; Scots law is one example. The difference is one day.
Why do other age calculators give a slightly different answer?
The usual differences are how month ends are handled (born on the 31st), whether the end date is counted as a full day, and whether a month is treated as 30 days. This calculator counts whole calendar months, then exact days, and never counts the end date twice.
How many days old am I?
The ‘In days’ line gives the exact number of calendar days from the date of birth to the chosen date, including every leap day. For the days between any two dates, use the days between dates calculator.
Can I work out an age on a date in the past or future?
Yes. Change the ‘Age on this date’ field to any date after the date of birth, for example to check an age on a start date, at retirement or on a historical event. The Today button puts back today’s date.
Sources
Last reviewed September 15, 2026