Temporal.PlainDate.prototype.eraYear

The eraYear accessor property of Temporal.PlainDate instances returns a non-negative integer representing the year of this date within the era, or undefined if the calendar does not use eras (e.g. ISO 8601). The year index usually starts from 1 (more common) or 0, and years in an era can decrease with time (e.g. Gregorian BCE). era and eraYear together uniquely identify a year in a calendar, in the same way that year does. It is calendar-dependent.

Unlike year, era and eraYear may change in the middle of a calendar year. For example, Japan started the Reiwa era on May 1, 2019, so dates from 2019-01-01 to 2019-04-30 have { era: "heisei", eraYear: 31 }, and dates from 2019-05-01 onwards have { era: "reiwa", eraYear: 1 }, but the year is always 2019 (because the Japanese calendar uses the ISO 8601 year as the default year).

The set accessor of eraYear is undefined. You cannot change this property directly. Use the with() method to create a new Temporal.PlainDate object with the desired new value.

const date = Temporal.PlainDate.from("2021-07-01"); // ISO 8601 calendar
console.log(date.eraYear); // undefined

const date2 = Temporal.PlainDate.from("2021-07-01[u-ca=gregory]");
console.log(date2.eraYear); // 2021

const date3 = Temporal.PlainDate.from("-002021-07-01[u-ca=gregory]");
console.log(date3.eraYear); // 2022; 0000 is used for the year 1 BC

const date4 = Temporal.PlainDate.from("2021-07-01[u-ca=japanese]");
console.log(date4.eraYear); // 3

const date = Temporal.PlainDate.from("2021-07-01[u-ca=gregory]");
const newDate = date.with({ era: "bc", eraYear: 100 });
console.log(newDate.toString()); // -000099-07-01[u-ca=gregory]

const date2 = Temporal.PlainDate.from("2021-07-01[u-ca=japanese]");
const newDate2 = date2.with({ era: "meiji", eraYear: 1 });
console.log(newDate2.toString()); // 1868-07-01[u-ca=japanese]