index.js
function guessEra(jdn, JD_AM = JD_EPOCH_OFFSET_AMETE_MIHRET, JD_AA = JD_EPOCH_OFFSET_AMETE_ALEM) {
  return (jdn >= (JD_AM + 365)) ? JD_AM : JD_AA;
}
function guessEra(jdn, JD_AM = JD_EPOCH_OFFSET_AMETE_MIHRET, JD_AA = JD_EPOCH_OFFSET_AMETE_ALEM) {
  return (jdn >= (JD_AM + 365)) ? JD_AM : JD_AA;
}
function guessEra ( jdn , JD_AM = JD_EPOCH_OFFSET_AMETE_MIHRET , JD_AA = JD_EPOCH_OFFSET_AMETE_ALEM ) { return ( jdn >= ( JD_AM + 365 ) ) ? JD_AM : JD_AA ; }
guesses ERA from JDN

@param  {Number} jdn
@return {Number}
guesses ERA from JDN