code: function (id, vals) {
        var custom = later.modifier[id];
        if(!custom) throw new Error('Custom modifier ' + id + ' not recognized!');
  
        modifier = id;
        values = arguments[1] instanceof Array ? arguments[1] : [arguments[1]];
        return this;
      }
docs: Custom modifier.
  
  recur().on(2011, 2012, 2013).custom('partOfDay');
  
  @api public