Get the date for all four advent.

advent.js

class Advent {
    #oneDayMs = 86400000;

    constructor(year = new Date().getFullYear()) {
        this.christmasEve = `${year}-12-24`;
        this.christmasEveDate = new Date(this.christmasEve);
    }

    #getDiff(days) {
        return this.christmasEveDate.getDay() + days;
    }

    #calcDate(days) {
        return +this.christmasEveDate - this.#getDiff(days) * this.#oneDayMs;
    }

    first() {
        return this.#calcDate(21);
    }

    second() {
        return this.#calcDate(14);
    }

    third() {
        return this.#calcDate(7);
    }

    fourth() {
        return this.#calcDate(0);
    }
}