docs: def _get_time(header, keys, name):
          """
          Get time from header
  
          Args:
              header (dict): Object header.
              keys (tuple of str): Header keys.
              name (str): Method name.
  
          Returns:
              float: The number of seconds since the epoch
          """
          for key in keys:
              try:
                  date_value = header.pop(key)
              except KeyError:
                  continue
              try:
                  # String to convert
                  return to_timestamp(parse(date_value))
              except TypeError:
                  # Already number
                  return float(date_value)
          raise UnsupportedOperation(name)