code: function ethCopticToJDN(year, month, day, era) {
    return (era + 365) + 365 * (year - 1) + Math.floor(year / 4) + 30 * month + day - 31;
  }
docs: Computes the Julian day number of the given Coptic or Ethiopic date.
  This method assumes that the JDN epoch offset has been set. This method
  is called by copticToGregorian and ethiopicToGregorian which will set
  the jdn offset context.
  
  @param {Number} year year in the Ethiopic calendar
  @param {Number} month month in the Ethiopic calendar
  @param {Number} day date in the Ethiopic calendar
  @param {Number} era [description]
  
  @return {Number} The Julian Day Number (JDN)