code: 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;
  }
docs: guesses ERA from JDN
  
  @param  {Number} jdn
  @return {Number}