jdn.js

// jdn conversions for date arithmetic/comparison
const intDiv = (l,r) => ~~(l/r); // integer division

const jdnFromIso = iso => {
  const [y, m, d] = iso.split('-').map(s => parseInt(s, 10));
  return intDiv(1461 * (y + 4800 + intDiv(m - 14, 12)), 4)
    + intDiv(367 * (m - 2 - 12 * intDiv(m - 14, 12)), 12)
    - intDiv(3 * intDiv(y + 4900 + intDiv(m - 14, 12), 100), 4)
    + d - 32075;
};

const isoFromJdn = jdn => {
  const f = jdn + 1401 + intDiv(intDiv(4 * jdn + 274277, 146097) * 3, 4) - 38;
  const e = 4 * f + 3;
  const g = intDiv(e % 1461, 4);
  const h = 5 * g + 2;
  const day = intDiv(h % 153, 5) + 1;
  const month = (intDiv(h, 153) + 2) % 12 + 1;
  const year = intDiv(e, 1461) - 4716 + intDiv(14 - month, 12);
  return year.toString().padStart(4, '0') + '-' + month.toString().padStart(2, '0') + '-' + day.toString().padStart(2, '0');
};