How years, months and days are counted
Years and days are unambiguous; months are not, because they have 28 to 31 days. This calculator counts a month as complete on the same day of the month as the birth. When that day does not exist — 31 June, or 29 February in a common year — the anniversary falls on the day after: 1 July, 1 March. It is the rule JavaScript’s own Date follows (MDN: 31 January plus one month is 2 March in a leap year), and it keeps the month count in step with the 1 March leap-day birthday described below.
Not every calculator does this. On 15 October 2026 we ran the same dates through six popular age calculators. Some count to the last day of a short month instead (as Python’s relativedelta does), some roll forward as this one does — and two showed negative days or “NaN” for a birthday on the 31st:
| Born → age on | This calculator | Calculators that use the month’s last day | Days (everyone agrees) |
|---|---|---|---|
| 31 Jan 2000 → 1 Mar 2026 | 26 years 0 months 29 days | 26 years 1 month 1 day | 9,526 |
| 29 Feb 2000 → 1 Mar 2026 | 26 years 0 months 0 days | 26 years 0 months 1 day | 9,497 |
| 31 Mar 2000 → 15 Mar 2026 | 25 years 11 months 12 days | 25 years 11 months 15 days | 9,480 |
Both conventions are defensible; what matters is using one consistently. When an exact figure matters, the total in days is the number no calculator disagrees on. And never divide days by 365: that drifts by a day for every leap year in the span — about ten days over 40 years. The Gregorian rule is that years divisible by 4 are leap years, except century years not divisible by 400, so 2000 was a leap year and 2100 will not be.
Birthdays on 29 February
A leap-day birthday exists once every four years — about 1 birth in 1,461. In the years between, the law that applies decides the date, and laws differ:
| Where | Birthday in a common year | Source |
|---|---|---|
| Scotland | 1 March | Age of Legal Capacity (Scotland) Act 1991, s.6(2) |
| Hong Kong | 1 March | Age of Majority (Related Provisions) Ordinance, Cap. 410, s.5(2) |
| Maryland (US) | 1 March | Senate Bill 86 of 2005 |
| Newfoundland and Labrador (Canada) | 28 February — “the last day of the month of February” | Age of Majority Act, s.3(2) |
| England and Wales; New Zealand | No specific rule | Family Law Reform Act 1969 s.9; Age of Majority Act 1970 s.5 — age is reached at the start of the birthday |
This calculator uses 1 March, so someone born on 29 February 2000 is still 25 on 28 February 2026 and turns 26 on 1 March. For anything legal, check the rule that governs the document in question.
The “day before your birthday” rule
Most people reach a new age on their birthday, but some systems count it from the day before. US Social Security states that “an individual attains a particular age on the first moment of the day preceding the anniversary of his/her birth” — its example: a person born on 1 January 1962 attains age 65 on 31 December 2026. That is why Social Security’s retirement-age table groups people born on 1 January with the previous year. Japan’s 1902 Act on Calculation of Age works the same way: the government confirmed in 2002 that age is added at midnight at the end of the day before the birthday, which is why a Japanese school year runs from those born on 2 April to those born on 1 April. English common law once did the same, until the Family Law Reform Act 1969 moved it to the birthday itself in England and Wales.
Age “as on” a date: exams, sport and school
Eligibility rules usually fix the age on one date. Put your date of birth in the calculator and the cut-off in “Age at date” — the quick buttons set 1 January, 1 July or 1 August. On your birthday you have attained that age.
- UPSC Civil Services 2026: “A candidate must have attained the age of 21 years and must not have attained the age of 32 years on the 1st of August, 2026 i.e., the candidate must have been born not earlier than 2nd August, 1994 and not later than 1st August, 2005.” Someone born on 1 August 1994 turns 32 on 1 August 2026 and is over the limit; born a day later, they are 31 years 11 months 30 days old on the cut-off and eligible. UPSC accepts the date of birth only as recorded in the matriculation or secondary-school certificate.
- NDA & NA (I) 2026 states a birth window instead: candidates “born not earlier than 01st Jul 2007 and not later than 01st July 2010”. For NDA (II) 2026 it is 1 January 2008 to 1 January 2011.
- Little League: a baseball player’s league age is their age on 31 August of the current year; for softball, their age on 31 December of the previous year.
Corrected age for babies born early
A premature baby’s development is measured from the due date, not the birth date. The American Academy of Pediatrics method: take the baby’s age in weeks since birth and subtract the weeks born early, which is 40 minus the weeks of pregnancy at birth. Its example: born at 32 weeks (8 weeks early) and now 16 weeks old gives a corrected age of 8 weeks. The Baby age tab does this from the due date or from the weeks of pregnancy.
- The CDC’s milestone checklists say to use corrected age if a child was born more than 3 weeks early.
- The ASQ screening questionnaires adjust “if he or she is 3 or more weeks premature and is under 2 years of chronological age”.
- The AAP describes using corrected age during the first 2 years; the UK’s RCPCH growth charts apply correction throughout childhood.
Chronological age for test scoring
Developmental and educational tests ask for the child’s chronological age, usually written years;months;days. Their manuals often describe a hand method that treats every month as 30 days. ASQ’s support team gives an example: born 18 February 2016 and tested on 4 April 2017, the real calendar gives 13 months 17 days, the 30-day hand method 13 months 16 days. This calculator shows both — the exact age, and the 30-day figure under it — and both reproduce ASQ’s example. Use whichever your test’s manual specifies, and check its rounding rule.
Korean, Japanese and Chinese age counting
South Korea used three ages: international age (만 나이, counted from 0 and increasing on each birthday), Korean age (세는 나이, 1 at birth and one more every 1 January) and year age (연 나이, the current year minus the birth year). Since 28 June 2023 the Civil Act and the Framework Act on Administrative Affairs make international age the legal default. Year age still applies where a law says so: the Juvenile Protection Act’s age for buying alcohol and tobacco, military service, and school entry. The calculator shows all three.
Japan told people to stop using traditional “counted years” (数え年) in its 1950 Act on the Way of Expressing Age. In China the traditional nominal age (虚岁) also starts at 1, but traditionally goes up with the lunar new year rather than on 1 January, so it needs a lunar calendar; this calculator does not compute it.
Retirement and pension ages by date of birth
| Rule | Age | Who |
|---|---|---|
| US Social Security full retirement age | 66 | Born 2 January 1943 to 1 January 1955 |
| 66 and 2 to 10 months | Born 2 January 1955 to 1 January 1960, rising two months a year | |
| 67 | Born 2 January 1960 or later (benefits can start from 62, reduced) | |
| US required minimum distributions (RMDs) | 73 | Born 1951–1958; 75 for those born 1960 or later |
| UK State Pension age | 66, rising to 67 | The rise to 67 happens between 2026 and 2028; from 6 March 1961 births, 67 |
Check the official calculators before acting: GOV.UK’s State Pension age checker and the Social Security Administration publish them.
Age in Excel, Python and JavaScript
- Excel:
=DATEDIF(A2,B2,"y")gives completed years and=DATEDIF(A2,B2,"ym")the months after them. Microsoft says it does not recommend the"md"unit, which “may result in a negative number, a zero, or an inaccurate result”;=B2-EDATE(A2,DATEDIF(A2,B2,"m"))gives the days instead. DATEDIF exists for compatibility with older Lotus 1-2-3 workbooks. - Python:
dateutil.relativedelta(today, birth)gives years, months and days; it moves a day past the end of a month back to the last day, so it agrees with the “month’s last day” column above. - JavaScript: store a date of birth as a plain date (2000-02-29), not a moment in time. Converting it to UTC moves it to the previous or next day in some time zones — the reason this calculator takes “today” from your own clock.
- On a basic calculator: subtract the birth year from the current year, then take off 1 if this year’s birthday has not happened yet.
Dogs and cats
The American Veterinary Medical Association says dogs “do not age at a rate of 7 human years per dog year”. A 2020 study of 104 Labrador retrievers (Wang et al., Cell Systems) matched DNA-methylation changes in dogs and people and proposed human age = 16 × ln(dog age) + 31. By that formula a 1-year-old Labrador is like a 31-year-old person, a 4-year-old like 53 and a 12-year-old like 71 — for Labradors; breeds age at different rates. The AVMA counts dogs as senior in the last quarter of their breed’s life expectancy (from 6–7 years for giant breeds), and cats “after 10 years of age”; the AAHA/AAFP guidelines call cats seniors from 10.
Birthday numbers
- 10,000 days is 27.38 years: someone born on 1 January 2000 reaches it on 19 May 2027.
- A billion seconds is 31.69 years.
- In England and Wales from 1995 to 2014, 26 September was the most common birthday — about 2,000 births a day against an average of 1,800 — and 8 of the 10 most common fell in late September (ONS).