Result
How to use this calculator
- Pick the year, month, and day you were born.
- Result updates to the current moment — refresh the page for an updated minute count.
- Day-of-week-born is calculated by the calendar, not approximated.
- To answer "how old was someone on a past or future date" (e.g. "how old was I on 9/11", "how old will my child be in 2030"), switch "Compute age as of" to "Pick a specific date" and set the as-of date.
- Future dates are not allowed — the calculator returns "—" if you enter one.
About this tool
Most "age calculators" stop at "you're 34 years old." The actual decomposition into years + months + days matters for forms (visa applications, insurance), milestones (a baby's first 18 months are usually tracked in days then weeks), and for the small joy of knowing exactly how many minutes you've been alive. The math has one trick: months don't have a fixed number of days. To handle this correctly we walk the calendar month by month rather than dividing by 30.44. The day-of-week-born is calculated using Zeller's congruence, the same formula used since the 1880s.
What this calculator does
This calculator returns your exact age in years, months, and days — plus the total counts in years, months, weeks, days, hours, and minutes lived since birth. It also names the day of the week you were born (using Zeller's congruence) and counts down to your next birthday, with the legally-conventional Feb-28 substitution applied for Feb-29 babies in non-leap years. The calendar math is calendar-aware: it borrows days from the previous month and months from the previous year, rather than dividing by an averaged 30.44 days.
How it works — the formula
Calendar age = years between birth date and today,
adjusting for whether this year's birthday has passed.
Total days = (today − birth_date) / 86,400,000 ms (leap years included automatically).Calendar-age arithmetic borrows days from the previous month and months from the previous year exactly as a human would on paper, so the result reads "years/months/days" rather than a decimal. The proleptic Gregorian calendar — applying Gregorian rules backward indefinitely — is the modern ISO 8601 default and is what most programming languages' Date class implements. Leap years follow the rule "divisible by 4, except century years not divisible by 400" (1900 was not a leap year; 2000 was).
Worked examples
- Inputs:
- birth = 1990-06-15, today = 2026-05-08
- Output:
- 35 years, 10 months, 23 days; 13,111 days; 314,664 hours
Birthday has not yet occurred this year — the borrow-from-previous-month step subtracts one month and pulls in April's 30 days to produce the 23-day remainder.
- Inputs:
- birth = 2000-02-29, today = 2026-03-01
- Output:
- 26 years, 0 months, 1 day (Feb 29 → Feb 28 in non-leap years per legal convention)
Most US states and the UK (Births and Deaths Registration Act 1953) treat the Feb-29 birthday as occurring on Feb 28 in non-leap years. The calculator mirrors that for the next-birthday countdown.
- Inputs:
- birth = 1985-07-20
- Output:
- Saturday (Zeller-formula or built-in Date.getDay() agree across implementations)
Zeller's congruence (1882) is a closed-form day-of-week formula. It matches JavaScript Date.getUTCDay() for all dates after the proleptic Gregorian start.
- Inputs:
- birth = 1955-02-24 (Steve Jobs), as-of = 2011-10-05 (the day he died)
- Output:
- 56 years, 7 months, 11 days
Set "Compute age as of" to "Pick a specific date" and enter 2011/10/5 to get the age on any past or future date.
What "age" actually means
Age is the elapsed time between a person's date of birth and a reference date (usually today). While the concept sounds simple, several conventions determine exactly how age is expressed — chronological age (calendar time elapsed), biological age (physiological state), and legal age (jurisdiction-specific birthday rules) can all diverge.
This calculator returns chronological age, computed as the difference between the reference date and the birth date, split into years / months / days. Because months have different lengths and years contain leap-day variability, the same "1 year and 3 months" might correspond to different total day counts depending on which months are involved.
How chronological age is calculated
The standard chronological age algorithm: start with (reference year − birth year), then subtract 1 if the reference date has not yet reached the birth month/day within the reference year. So a person born June 15 1990 checking on January 1 2026 is 35 years old (2026 − 1990 = 36, minus 1 because January 1 is before June 15). On June 16 2026 the same person becomes 36.
This calculator additionally breaks the residual into complete months and remaining days, which is useful for infant and toddler tracking (where age is commonly expressed in months) and for legal cutoffs that reference exact months.
The February 29 birthday case
People born on February 29 have birthdays that only occur every four years. Common law and most jurisdictions treat their "birthday" on non-leap years as March 1 (the day after the actual birth date). For legal age purposes — voting age, drinking age, driving age — the birthday is deemed to occur on March 1 in non-leap years.
This calculator handles Feb 29 birthdays consistently: age increments on Feb 28 in non-leap years for algorithmic simplicity. Some legal jurisdictions instead increment on March 1. For high-stakes legal age determinations involving Feb 29 birthdays, defer to the specific jurisdiction's statute rather than a calculated result.
Age in months, weeks, and days
For infants, age is conventionally expressed in months (a "6-month-old baby"). Once past age 2 or so, years become the default unit. For legal proceedings, insurance actuarial tables, and some medical protocols, age in days is used for maximum precision — a person is 12,775 days old at age 35.
The calculator returns all three representations. Age in days is calculated as (reference date − birth date) / 86,400,000 milliseconds, rounded to the nearest whole day. Age in weeks is age in days / 7. Age in months is a slightly complex operation because months vary in length; the standard approach is to count whole calendar months elapsed, plus a fractional part.
Chronological vs biological age
Chronological age is a poor proxy for physiological state past middle age. Two 60-year-olds can have dramatically different biological ages measured by cardiovascular health, cognitive function, physical strength, and cellular markers like telomere length or epigenetic clock methylation patterns.
Biological age tests measure things like DNA methylation (Horvath clock, GrimAge, PhenoAge), plasma protein signatures, physical fitness testing, and cognitive assessment. Well-validated biological age scores can be 5-15 years higher or lower than chronological age depending on lifestyle factors — smoking, diet, exercise, sleep, and stress all contribute.
Chronological age is what you see reported on birthdays and passports; biological age is what determines healthspan (the number of remaining healthy years). This calculator returns chronological age; biological age requires lab tests and specific validated assessments.
Legal age milestones (US default)
Common US legal age thresholds referenced by this calculator's "next milestone" feature:
- 16: minimum legal driving age in most US states (varies 14-17)
- 17: minimum age for R-rated movies without a guardian; military enlistment with parental consent
- 18: legal adulthood — voting, contracts, marriage in most states, jury duty, military enlistment
- 21: legal drinking age in all US states (federal Minimum Drinking Age Act 1984)
- 25: minimum age for full car-rental with no surcharge at most agencies; House of Representatives eligibility
- 30: US Senate eligibility
- 35: US Presidency eligibility
- 50: AARP eligibility; catch-up 401(k) contributions begin
- 62: earliest Social Security retirement benefits
- 65: Medicare eligibility
- 67: Full Social Security retirement age (for those born 1960 or later)
- 73: Required Minimum Distributions begin for Traditional 401(k) and IRA (SECURE 2.0 Act)
International legal ages differ significantly. Legal drinking age is 18 in most of Europe, 16 in some countries. Age of majority is 20 in Japan (recently lowered from 20 to 18 in 2022), 21 in Singapore. Always check the specific jurisdiction for legal age questions.
The calendar quirks age calculation handles
Leap years occur every 4 years except century years not divisible by 400 (so 1900 was NOT a leap year, 2000 WAS). Between 1900 and 2100 the leap-year rule is simply "divisible by 4" because 2000 was a leap year — but pre-1900 and post-2100 dates need the full rule. This calculator uses the JavaScript Date object, which correctly handles the full Gregorian calendar.
The Gregorian calendar was adopted in 1582 in Catholic countries and gradually thereafter elsewhere (Britain and colonies switched in 1752, Russia in 1918). Age calculations across the "gap" (October 5-14 1582 in Catholic countries, September 3-13 1752 in Britain) can produce unexpected results, but these dates are historically rare enough to not affect modern users.
Astronomical time keeping uses different definitions still — Julian dates (continuous days since noon January 1 4713 BCE), and Unix timestamps (seconds since January 1 1970 UTC). These are useful for scientific time-arithmetic but rarely used for calendar-age purposes.
When to use this vs other tools
Age Calculator is the everyday tool. For pregnancy-specific or sleep-window calculations, a more targeted tool is faster.
- Due Date Calculator
Use for pregnancy — Due Date Calculator applies Naegele's rule (LMP + 280 days) and returns the trimester schedule, which is unrelated to post-natal age math.
- Sleep Calculator
Use for sleep planning — Sleep Calculator schedules 90-minute REM cycles around a target bedtime or wake-time, a separate calendar-math problem from age accumulation.
- BMI Calculator
Use to interpret what your age implies for healthy weight — adult BMI bands (18.5–24.9) hold across ages, but the National Institute on Aging notes higher BMI may be appropriate over age 65.
- Calorie Calculator (TDEE)
Use because BMR and TDEE drop ~1–2% per decade after 30. Plug your exact age into TDEE rather than rounding.
Authority note
ISO 8601 defines the proleptic Gregorian calendar (Gregorian rules applied backward indefinitely) that every modern programming language's date library implements. NIST is the US authoritative timekeeping reference. This calculator uses both — so its output matches what a standard-compliant library on any platform would return.
Limitations
- Pre-Gregorian dates use the proleptic Gregorian convention; historical sources may use Julian and so disagree by up to 10–13 days.
- Time-zone handling: a birth recorded at 11 pm in one zone is "yesterday's" date in another. This calculator uses the browser's local time zone.
- Future birth dates (typo or pregnancy planning) return "—" rather than a negative age.
- Daylight-saving transitions and leap seconds are absorbed into the millisecond-based total-days count and have no observable effect on years/months/days output.
Age calculations are exact within the millisecond precision of the JavaScript Date object; legal age determinations should reference your jurisdiction's specific birthday-handling rules (especially for Feb 29 leap-day birthdays).