docs: def _validate_date_str(str_):
      """Validate str as a date and return string version of date"""
  
      if not str_:
          return None
  
      # Convert to datetime so we can validate it's a real date that exists then
      # convert it back to the string.
      try:
          date = datetime.strptime(str_, DATE_FMT)
      except ValueError:
          msg = 'Invalid date format, should be YYYY-MM-DD'
          raise argparse.ArgumentTypeError(msg)
  
      return date.strftime(DATE_FMT)