# 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.

- Calculator id: `age` · Category: Everyday Life (`everyday`) · Tool name: `calculate_age`
- Canonical page: https://tttkmbb.com/everyday/age · This document: https://tttkmbb.com/everyday/age.md · JSON definition: https://tttkmbb.com/everyday/age.json

## Purpose

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.

**Use when:** 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).

## Input

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

## Output

| Field | 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. |

## 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.

## Data Sources

- Gregorian calendar – leap-year rule and month lengths (Wikipedia) — https://en.wikipedia.org/wiki/Gregorian_calendar (reference, retrieved 2026-09-23)
- ISO 8601 – Date and time format (Wikipedia) — https://en.wikipedia.org/wiki/ISO_8601 (reference, retrieved 2026-09-23)

Data freshness: `daily` (max_age_seconds 86400). Depends on the current date unless the date is supplied.

## API

- `GET https://tttkmbb.com/api/v1/calculate/age?birth_date=…`
- `POST https://tttkmbb.com/api/v1/calculate/age` with JSON body `{"inputs": {…}}`
- Response: unified envelope (`success`, `request`, `result.values`, `result.units`, `sources`, `freshness`, `timestamp`, `next_actions`, `links`); see https://tttkmbb.com/docs/response-format.md
- Schema: https://tttkmbb.com/api/v1/calculators/age · OpenAPI operationId `calculate_age` in https://tttkmbb.com/openapi.json
- Authentication: none. Rate limit: fair use, see https://tttkmbb.com/docs/rate-limits.md.

## MCP

- Server: `https://tttkmbb.com/mcp` (Streamable HTTP, JSON-RPC 2.0, no auth)
- Tool: `calculate_age` (dedicated) or `run_calculator` with `{"calculator_id": "age", "inputs": {…}}`

## Example

- Born 1990-05-17, as of 2026-09-23: inputs `{"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: inputs `{"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
```

## Limitations

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). 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. All values are computed from the formula above; no measurement or live data is involved.

## 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

- [Date Difference Calculator](https://tttkmbb.com/everyday/date-difference.md) — Difference between any two dates, including business days.
- [Days Until Calculator](https://tttkmbb.com/everyday/days-until.md) — Countdown to any date, e.g. the next birthday.
- [Day of the Week Calculator](https://tttkmbb.com/everyday/day-of-week.md) — Weekday and ISO week of any date.