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Age Calculator

Computes the exact age on a given date as years, months and days by Gregorian calendar arithmetic, plus total days and weeks, the weekday of birth, and the date of and days until the next birthday.

When to use

You need someone's exact age on a date, the total number of days lived, the weekday they were born on, or how many days remain until their next birthday.

Do not use when: You need the difference between two arbitrary dates with a business-day count (use date-difference) or a countdown to an event (use days-until).

Formula

years, months, days = calendar difference birth_date → as_of_date (when the day-of-month is smaller, days are borrowed from the month before as_of_date); total_days = as_of_date − birth_date; total_weeks = total_days / 7; age_decimal_years = total_days / 365.2425; next_birthday = first (month, day) of birth on or after as_of_date

Uses the proleptic Gregorian calendar in UTC; no time zones or times of day are considered. A person born on 29 February is treated as having their birthday on 1 March in common years.

Inputs

Parameter	Type	Unit	Required	Description
`birth_date`	date		yes	Date of birth (ISO 8601, YYYY-MM-DD).
`as_of_date`	date		default today	Date on which to evaluate the age; defaults to the current UTC date.

Outputs

Output	Type	Unit	Description
`years`	integer		Completed years.
`months`	integer		Completed months beyond the years (0–11).
`days`	integer		Days beyond the completed months.
`age_text`	string		Age written as years, months and days (zero parts omitted).
`total_days`	integer	days	Days from birth_date to as_of_date.
`total_weeks`	number	weeks	total_days / 7.
`age_decimal_years`	number	years	total_days / 365.2425 (mean Gregorian year).
`next_birthday_date`	date		First birthday anniversary on or after as_of_date (a 29 February birthday falls on 1 March in non-leap years).
`days_until_next_birthday`	integer	days	0 on the birthday itself.
`day_of_week_born`	string		Day of the week of birth_date.

Example

Born 1990-05-17, as of 2026-09-23: {"birth_date":"1990-05-17","as_of_date":"2026-09-23"} → {"years":36,"months":4,"days":6,"age_text":"36 years 4 months 6 days","total_days":13278,"total_weeks":1896.9,"age_decimal_years":36.35,"next_birthday_date":"2027-05-17","days_until_next_birthday":236,"day_of_week_born":"Thursday"}

Born 1985-12-25, as of 2026-01-10: {"birth_date":"1985-12-25","as_of_date":"2026-01-10"} → {"years":40,"months":0,"days":16,"age_text":"40 years 16 days","total_days":14626,"next_birthday_date":"2026-12-25","days_until_next_birthday":349,"day_of_week_born":"Wednesday"}

GET https://tttkmbb.com/api/v1/calculate/age?birth_date=1990-05-17&as_of_date=2026-09-23

Machine access

API: GET https://tttkmbb.com/api/v1/calculate/age (query parameters) or POST with a JSON body {"inputs": {...}}
Schema: https://tttkmbb.com/api/v1/calculators/age · Markdown: https://tttkmbb.com/everyday/age.md · JSON definition: https://tttkmbb.com/everyday/age.json
MCP: server https://tttkmbb.com/mcp, tool calculate_age
OpenAPI operationId: calculate_age
Freshness: daily. Authentication: none. Rate limit: fair use (see rate limits).

Sources

Gregorian calendar – leap-year rule and month lengths (Wikipedia) (reference)
ISO 8601 – Date and time format (Wikipedia) (reference)

FAQ

How are months counted when the day of the month does not fit?

Whole months are counted first; the leftover days are borrowed from the month preceding as_of_date. Born 31 January, on 1 March the age is 1 month 1 day (February has 28 or 29 days).

What happens with a 29 February birthday?

In common years the birthday is taken as 1 March, so days_until_next_birthday counts to 1 March; in leap years it counts to 29 February.

Related calculators

Date Difference Calculator — Difference between any two dates, including business days.
Days Until Calculator — Countdown to any date, e.g. the next birthday.
Day of the Week Calculator — Weekday and ISO week of any date.