Age

Age in months and years, age corrected for prematurity, and age formatted for charts.

Chronological: 11.3 months
Corrected: 9.5 months
Completed years: 0

Installation

bunx shadcn@4.22.1 add https://organui.com/r/age.json

This adds lib/age.ts, with no dependencies.

Usage

import { ageInMonths, correctedAgeInMonths, formatAge } from "@/lib/age"

const birth = new Date("2025-02-10")
const measured = new Date("2026-01-20")

formatAge(ageInMonths(birth, measured)) // "11.3 months"
// Born at 32 weeks: eight weeks come off.
formatAge(correctedAgeInMonths(birth, measured, 32)) // "9.5 months"

API Reference

Age exports
Export	Description
`ageInMonths(birth, measured)`	Age in months, counting the average month of 30.4375 days as the WHO standards do.
`ageInYears(birth, on)`	Completed years, as a birthday counts them, in UTC.
`correctedAgeInMonths(birth, measured, gestationalWeeks)`	Age less the weeks born before 40, never below zero. Growth charts use it for babies born before 37 weeks, usually until 24 months.
`formatAge(ageMonths)`	Months to one decimal under 24 months, such as "9.5 months", then years and months, such as "3 y 2 mo".

Notes

Pass dates at UTC midnight, such as new Date("2025-02-10"), so ages don't shift with the reader's time zone.

The growth chart uses this file for its ages.