Exact age, down to the minute
Exact age, down to the minute
Calculating age sounds trivial and is not, because months have different lengths and years are not all the same size. The correct method works down from the largest unit.
Subtract the birth year from the current year. If the current month and day have not yet reached the birth month and day, subtract one year — the birthday has not happened yet this year. Then count the completed months since the last birthday, and the remaining days after that.
This is why age cannot be found by dividing total days by 365. That approach drifts by roughly a day every four years and produces visibly wrong answers over a lifetime.
Earth's orbit takes about 365.2422 days, not 365. Without correction, the calendar would drift against the seasons by roughly 24 days per century.
The Gregorian rule handles this with three conditions: a year is a leap year if divisible by 4, unless divisible by 100, unless also divisible by 400. So 2000 was a leap year and 1900 was not. This gives an average year of 365.2425 days — accurate to within about one day every 3,200 years.
People born on 29 February have a birthday only in leap years. Most jurisdictions treat 1 March as the legal anniversary in common years, though some use 28 February. It varies by country and by the specific legal context.
The system most of the world uses — where you are 0 at birth and gain a year on each birthday — is not universal.
The traditional East Asian age reckoning counted a newborn as one year old and added a year at Lunar New Year rather than on the birthday, so a baby born in December could be two years old weeks later. South Korea used a version of this in everyday life until officially standardising on international age in 2023.
Insurance underwriting sometimes uses age nearest birthday, rounding to the closest one rather than the last. Someone 34 years and 8 months old is treated as 35, which can affect premiums.
Gestational age in medicine is counted from the last menstrual period rather than conception, making it roughly two weeks longer than the actual age of the embryo — a convention that regularly confuses people encountering it for the first time.
Legal thresholds. Voting, driving, contracts, and alcohol purchase all turn on exact date, and in most jurisdictions you attain a given age at the start of your birthday.
Medicine. Paediatric dosing is frequently age-banded, and developmental milestones are assessed against exact age in months. For premature infants, corrected age — adjusted for early birth — is used for the first two years.
Sport. Age-group eligibility usually uses a cut-off date rather than the competition date, which is why children born just after a cut-off can be nearly a year older than their youngest competitors.
Employment and pensions. Retirement eligibility, benefit calculations, and service length all depend on precise dates rather than approximate years.
Seeing an age expressed in days, hours, or minutes tends to land differently than seeing it in years — 30 years is about 10,950 days, and roughly 262,800 hours. The figures are exact but not more meaningful, and it is worth noticing that the large numbers can feel weightier than the small ones without carrying any additional information.
Subtract the birth year from the current year, then subtract one if your birthday has not yet occurred this year. Count completed months since your last birthday, then the remaining days. Dividing total days by 365 gives a wrong answer because of leap years.
A year is a leap year if divisible by 4, unless it is divisible by 100, unless it is also divisible by 400. So 2000 was a leap year and 1900 was not.
In leap years the date exists. In common years, most jurisdictions treat 1 March as the legal anniversary, though some use 28 February. The convention varies by country and legal context.
No. Traditional East Asian reckoning counted a newborn as one year old and added a year at Lunar New Year. South Korea used a version of this in daily life until officially adopting international age in 2023.