Date.year

Date is an immutable proleptic Gregorian calendar date from 0001-01-01 through 9999-12-31. It contains no time of day, time zone, or locale. The same calendar rules apply before and after 1582.

Date.fromYmd accepts years from 1 through 9999 and months from 1 through 12. The day must be a real day of the selected month and year. All range endpoints are included.

A year divisible by 4 is a leap year except a century year that is not divisible by 400. February has 29 days in a leap year and 28 otherwise. April, June, September, and November have 30 days. The other months have 31.

Valid components return Ok containing that date. Year zero, a year outside the supported range, an invalid month, and an invalid day return Err containing a descriptive nonempty string. Components are not normalized into another date.

The error string describes the failed construction and has no stable machine readable format. Callers branch on Ok or Err.

Date.year, Date.month, and Date.day are calls with no arguments. They return the validated components of the receiver. A valid Date makes these observations total and leaves the receiver unchanged.

Arguments are evaluated once in source order before validation. An earlier fault prevents later evaluation. Constructors and observations do not read a host clock or perform external effects.