code: function (id) {
        var custom = later[id];
        if(!custom) throw new Error('Custom time period ' + id + ' not recognized!');
  
        add(id, custom.extent(new Date())[0], custom.extent(new Date())[1]);
        return this;
      }
docs: Custom time period.
  
  recur().on(2011, 2012, 2013).customPeriod('partOfDay');
  
  @api public