const hoursperday = 24
const minutesperhour = 60
const secondsperminute = 60
const secondms = 1000
const minutems = secondms * secondsperminute
const hourms = minutems * minutesperhour
const dayms = hourms * hoursperday

const solaryear = 365.242

const yeardays = 366
const leapchange = -6
const leapyeardays = yeardays + leapchange

const monthdays = 30

const leapfreq = 8

const exleapcyclelength = 800;
const extraleapyear = 500;

const exleapcycledays = (exleapcyclelength * yeardays) + leapchange + ((exleapcyclelength / leapfreq) * leapchange)

const leapcycledays = (leapfreq * yeardays) + leapchange;

const unityzero = Date.parse("2000-03-08T00:00:00.000Z");

function unixtounity(unixtime) {
    const unitytime = unixtime - unityzero;
    const days = unitytime / dayms;

    const exleapcycles = Math.floor(days / exleapcycledays);

    const exleapcycleprogress = days - exleapcycles * exleapcycledays;

    const leapcycles = Math.floor(exleapcycleprogress / leapcycledays);

    const leapcycleprogress = exleapcycleprogress - leapcycles * leapcycledays;

    let years;

    let yearprogress;

    if (leapcycleprogress > leapyeardays) {

        years = Math.floor((leapcycleprogress - leapyeardays) / yeardays) + 1;

        yearprogress = leapcycleprogress - ((years * yeardays) + leapchange);

    } else {

        years = 0;

        yearprogress = leapcycleprogress;

    }

    const months = Math.floor(yearprogress / monthdays);

    const monthprogress = yearprogress - months * monthdays;

    const date = Math.floor(monthprogress);

    return `${12000 + (exleapcycles * exleapcyclelength) + (leapcycles * leapfreq) + years}-${months + 1}-${date + 1}`

}

const oldtimes = new Date();
oldtimes.setYear(-600)

console.log(unixtounity(Date.now()))
console.log(unixtounity(Date.parse("3500-03-03")))
console.log(unixtounity(Date.parse("0010-12-30")))
console.log(unixtounity(oldtimes.getTime()))