code: def get_series_as_of_date(self, series_id, as_of_date):
          
          as_of_date = pd.to_datetime(as_of_date)
          df = self.get_series_all_releases(series_id)
          data = df[df['realtime_start'] <= as_of_date]
          return data
docs: Get latest data for a Fred series id as known on a particular date. This includes any revision to the data series
          before or on as_of_date, but ignores any revision on dates after as_of_date.
  
          Parameters
          ----------
          series_id : str
              Fred series id such as 'GDP'
          as_of_date : datetime, or datetime-like str such as '10/25/2014'
              Include data revisions on or before this date, and ignore revisions afterwards
  
          Returns
          -------
          data : Series
              a Series where each index is the observation date and the value is the data for the Fred series