Converts a Temporal.Instant to a Julian Date. Formatted for readability.

instant-to-jd.js

const hoursInDay        = 24
const minutesInDay      = 1440
const secondsInDay      = 86400
const millisecondsInDay = 86400000
const microsecondsInDay = 86400000000
const nanosecondsInDay  = 86400000000000

/**
 * Converts a Temporal.Instant to a Julian Date (decimal number).
 **/
function instantToJD(instant) {
  let {
    year,
    month,
    day,
    hour,
    minute,
    second,
    millisecond,
    microsecond,
    nanosecond,
  } = instant.toZonedDateTimeISO('UTC')
  
  // *waves wand*
  let a = Math.floor((14 - month) / 12)
  let y = year + 4800 - a
  let m = month + 12 * a - 3
  let jdn = day + Math.floor((153 * m + 2) / 5)
    + 365 * y + Math.floor(y / 4)
    - Math.floor(y / 100)
    + Math.floor(y / 400)
    - 32045

  let fraction = hour / hoursInDay
    + minute / minutesInDay
    + second / secondsInDay
    + millisecond / millisecondsInDay
    + microsecond / microsecondsInDay
    + nanosecond / nanosecondsInDay

  return jdn - 0.5 + fraction
}

let jd = instantToJD(Temporal.Now.instant())
console.log(jd)