dates.age

pub fn age(birth_date: &str, on_date: &str) -> i64

fune!(dates.age@^1);  // then call age(…)

use super::funejson::Value;  ← the fune runtime: the JSON value the test vectors use; fune build keeps it only where a signature takes one
use super::dates_add_days::{is_leap_year, parse_iso_date};  ← from dates.add-days ^1.0.0 · built alongside by fune

/// Completed years on `on_date`. A person attains an age at the start of their
/// birthday; for a 29 February birthday in a year without one, the birthday is
/// taken to be 1 March (Age of Legal Capacity (Scotland) Act 1991 s.6(2), and
/// the UK reading of Family Law Reform Act 1969 s.9), not 28 February.
///
/// # Panics
/// Panics on a malformed or impossible date, or `on_date` before `birth_date`.
pub fn age(birth_date: &str, on_date: &str) -> i64 {
    let birth = parse_iso_date(birth_date);
    let on = parse_iso_date(on_date);
    if on_date < birth_date {
        panic!("onDate {} is before birthDate {}", on_date, birth_date);
    }
    let (mut birthday_month, mut birthday_day) = (birth.month, birth.day);
    if birth.month == 2 && birth.day == 29 && !is_leap_year(on.year) {
        birthday_month = 3;
        birthday_day = 1;
    }
    let mut years = on.year - birth.year;
    if (on.month, on.day) < (birthday_month, birthday_day) {
        years -= 1;
    }
    years
}

pub fn fune_vector(args: &[Value]) -> Value {
    Value::Int(age(args[0].as_str(), args[1].as_str()))
}

Install

fune build

With that line in your source, in a Rust project (language rust in fune.project), fune build resolves it and its 1 dependency, pins them in fune.lock, downloads only the Rust 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. A crate’s build.rs runs it before every compile. Or pin a range in fune.project and build in one step:

fune add dates.age

The manifest, vectors and README with only the Rust implementation. Install it without the registry with fune add ./dates.age-1.0.0-rust.fune, or fetch it from a terminal with fune pull dates.age@1.0.0:rust.

The whole function, every language, is one file too: dates.age-1.0.0.fune, 8,959 bytes, sha256 55db52a3f8f0f2ddc2ef538a47e196d364d089b870e596f48b02fe9348c6e070. It installs into a project of any language.

// fune: before dates.age

// fune: after dates.age

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

// fune: step dates.age after <n|label>

More from the author

- Scotland states it outright. Age of Legal Capacity (Scotland) Act 1991, section 6: "(1) The time at which a person attains a particular age expressed in years shall be taken to be the beginning of the relevant anniversary of the date of his birth. (2) Where a person has been born on 29th February in a leap year, the relevant anniversary in any year other than a leap year shall be taken to be 1st March." https://www.legislation.gov.uk/ukpga/1991/50/section/6 - England, Wales and Northern Ireland: Family Law Reform Act 1969, section 9(1), "The time at which a person attains a particular age expressed in years shall be the commencement of the relevant anniversary of the date of his birth." https://www.legislation.gov.uk/ukpga/1969/46/section/9 . The Act does not name the anniversary of a 29 February birth; 1 March is the reading applied in practice and the one the Scottish Act writes down.

Some other jurisdictions and systems use 28 February instead. The difference is one day every three years in four, and it is exactly the day an age-gated sale or a pension start date is checked on, so this capability picks the UK rule and says so rather than leaving it to whatever a naive clamp does.

A measuring date before the birth date is an error rather than a negative or zero age: it is always a data or argument-order mistake.

Files