def FirstEventTimestamp(self):
    """Returns the timestamp in seconds of the first event.

    If the first event has been loaded (either by this method or by `Reload`,
    this returns immediately. Otherwise, it will load in the first event. Note
    that this means that calling `Reload` will cause this to block until
    `Reload` has finished.

    Returns:
      The timestamp in seconds of the first event that was loaded.

    Raises:
      ValueError: If no events have been loaded and there were no events found
      on disk.
    """
    if self._first_event_timestamp is not None:
      return self._first_event_timestamp
    with self._generator_mutex:
      try:
        event = next(self._generator.Load())
        self._ProcessEvent(event)
        return self._first_event_timestamp

      except StopIteration:
        raise ValueError('No event timestamp could be found')