health.age-in-months Unreviewed

export function ageInMonths(birthDate: string, onDate: string): AgeInMonths

/** A child's age as completed months plus days, and the days since birth. */
export interface AgeInMonths {
  /** completed calendar months since birth */
  readonly months: number;
  /** days since the last completed month, 0 or more */
  readonly days: number;
  /** days since birth; 0 on the day of birth */
  readonly totalDays: number;
}

import { ageInMonths } from "#fune/health.age-in-months@^1";

import { addDays, daysBetween, parseIsoDate } from "./dates_add_days.ts";  ← from dates.add-days ^1.0.0 · built alongside by fune
import { addMonths } from "./dates_add_months.ts";  ← from dates.add-months ^1.0.0 · built alongside by fune
import { type AgeInMonths } from "./health_age_in_months_types.ts";

/**
 * The day the given number of months is completed. addMonths clamps 31 January
 * + 1 month to 28 February; a clamped day has not yet reached the birth day, so
 * the month completes the day after it (1 March), the same rule dates.age uses
 * for a 29 February birthday.
 */
function monthAnniversary(birthDate: string, birthDay: number, months: number): string {
  const shifted = addMonths(birthDate, months);
  return parseIsoDate(shifted).day < birthDay ? addDays(shifted, 1) : shifted;
}

/** Completed months and remaining days from birthDate to onDate. */
export function ageInMonths(birthDate: string, onDate: string): AgeInMonths {
  const birth = parseIsoDate(birthDate);
  const on = parseIsoDate(onDate);
  if (onDate < birthDate) {
    throw new RangeError(`onDate ${onDate} is before birthDate ${birthDate}`);
  }
  let months = (on.year - birth.year) * 12 + (on.month - birth.month);
  let anniversary = monthAnniversary(birthDate, birth.day, months);
  if (anniversary > onDate) {
    months -= 1;
    anniversary = monthAnniversary(birthDate, birth.day, months);
  }
  return { months, days: daysBetween(anniversary, onDate), totalDays: daysBetween(birthDate, onDate) };
}

Install

fune build

With that line in your source, in a TypeScript project (language typescript in fune.project), fune build resolves it and its 2 dependencies, pins them in fune.lock, downloads only the TypeScript package of each, and builds the code above into your project’s .fune/build, one readable file per capability with a header linking back here. Or pin a range in fune.project and build in one step:

fune add health.age-in-months

The manifest, vectors and README with only the TypeScript implementation. Install it without the registry with fune add ./health.age-in-months-1.0.1-typescript.fune, or fetch it from a terminal with fune pull health.age-in-months@1.0.1:typescript.

The whole function, every language, is one file too: health.age-in-months-1.0.1.fune, 13,308 bytes, sha256 6ba2f856fa499229596115645b913970f63d353863d146ba2c24df0dcfff919e. It installs into a project of any language.

// fune: before health.age-in-months

// fune: after health.age-in-months

// fune: replace dates.add-days in health.age-in-months
// fune: replace dates.add-months in health.age-in-months

// fune: step health.age-in-months after <n|label>

More from the author

Months are always counted from the date of birth, not stepped a month at a time, so a 31st-of-the-month birthday does not decay to the 28th after February: born 31 January, 2 months are completed on 31 March.

A measuring date before the birth date is an error, not a negative age. Dates are ISO `YYYY-MM-DD`; an impossible date (2026-02-30) or a UK-style `15/05/2026` is an error.

Built on `dates.add-months` and `dates.add-days`; no clinical thresholds are applied here.

## Before you rely on this

**Not professional advice.** This capability calculates health figures from published rules. It is a software component for developers, not medical advice. Rules change and every rate here has an effective date. Check that the dates cover your case. Verify results against the official sources listed above, and have a clinician review how you use it, before anyone relies on the output. Provided "as is" under its licence, without warranty.

**Not a medical device.** It is not intended to diagnose, treat or support clinical decisions about any individual. Anyone building it into clinical software is responsible for that software's regulatory status, and must validate it under their own clinical governance.

**Unreviewed.** This capability's implementations agree in every language and pass its published test vectors, which were worked out from the official sources cited. But no qualified clinician has yet checked those vectors, or confirmed that the capability covers the cases it claims. Treat it as a draft. Do not use it for real people, money or decisions without your own expert review. Once a qualified reviewer signs off, this notice is replaced with their name, qualification and the date. Each new version needs fresh sign-off.

1.0.1 marks it unreviewed. The code and the tests are unchanged.

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