code: function jdnToEthiopic(jdn, era = JD_EPOCH_OFFSET_AMETE_MIHRET) {
    const r = mod((jdn - era), 1461);
    const n = mod(r, 365) + 365 * Math.floor(r / 1460);
  
    const year = 4 * Math.floor((jdn - era) / 1461) + Math.floor(r / 365) - Math.floor(r / 1460);
    const month = Math.floor(n / 30) + 1;
    const day = mod(n, 30) + 1;
  
    return { year, month, day };
  }
docs: given a JDN and an era returns the Ethiopic equivalent
  
  @param  {Number} jdn
  @param  {Number} era
  @return {Object} { year, month, day }