dates.age

def age(birth_date: str, on_date: str) -> int

from fune.dates.age import age  # dates.age@^1

from .dates_add_days import is_leap_year, parse_iso_date  ← from dates.add-days ^1.0.0 · built alongside by fune


def age(birth_date: str, on_date: str) -> int:
    """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.
    """
    birth = parse_iso_date(birth_date)
    on = parse_iso_date(on_date)
    if on_date < birth_date:
        raise ValueError("onDate %s is before birthDate %s" % (on_date, birth_date))
    birthday_month, birthday_day = birth.month, birth.day
    if birth.month == 2 and birth.day == 29 and not is_leap_year(on.year):
        birthday_month, birthday_day = 3, 1
    years = on.year - birth.year
    if (on.month, on.day) < (birthday_month, birthday_day):
        years -= 1
    return years

Install

fune build

With that line in your source, in a Python project (language python in fune.project), fune build resolves it and its 1 dependency, pins them in fune.lock, downloads only the Python 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 dates.age

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

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