code: function listTimezones () {
    if (arguments.length == 0) {
      throw new Error("You must set a callback");
    }
    if (typeof arguments[arguments.length - 1] != "function") {
      throw new Error("You must set a callback");
    }
    var cb = arguments[arguments.length - 1]
      , subset = (arguments.length > 1 ? arguments[0] : null)
  
    return listTimezonesFolder(subset ? subset + "/" : "", subset ? path.join(TZDIR, "/" + subset) : TZDIR, function (err, tzs) {
      if (err) return cb(err);
      cb(null, tzs.sort());
    });
  }
docs: Lists the timezones that the current system can accept. It does this by going
  on a recursive walk through the timezone dir and collecting filenames.