# health.age-in-months

A child's age as completed months plus days on a given date: born 10 March
2026, on 25 May 2026 the child is 2 months 15 days. Paediatric growth charts,
immunisation schedules and many dosing tables are read in months, and the
date to measure on is an argument, never the clock. `totalDays` is the age in
days as well, for neonates.

**When a month is completed.** A month is completed on the same day of the
month as the birth. When that day does not exist in the month (born 31
January; born 29 February in a common year), the month is completed on the
first day of the following month, not on the last day of the short month.
Born 31 January 2026, the child is 0 months 28 days on 28 February and
1 month 0 days on 1 March. This is the same rule `dates.age` applies to a
29 February birthday (the anniversary is 1 March), so a child born 29
February 2024 is 11 months 30 days on 28 February 2025 and 12 months, and
one year old, on 1 March 2025. A naive "add a month and clamp" says the
child born on 31 January is a month old on 28 February, three days early.

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.