docs: def __validate_dates(start_date, end_date):
      """Validate if a date string.
  
      Validate if a string is a date on yyyy-mm-dd format and it the
      period between them is less than a year.
      """
      try:
          start_date = datetime.datetime.strptime(start_date, '%Y-%m-%d')
          end_date = datetime.datetime.strptime(end_date, '%Y-%m-%d')
      except ValueError:
          raise ValueError("Incorrect data format, should be yyyy-mm-dd")
      if (end_date - start_date).days > 366:
          raise ValueError("The difference between start and end date " +
                           "should be less than or equal to 366 days.")
      if (end_date - start_date).days < 0:
          raise ValueError("End date cannot be before start date.")