def _GetNormalizedTimestamp(self):
    """Retrieves the normalized timestamp.

    Returns:
      float: normalized timestamp, which contains the number of seconds since
          January 1, 1970 00:00:00 and a fraction of second used for increased
          precision, or None if the normalized timestamp cannot be determined.
    """
    if self._normalized_timestamp is None:
      if (self._timestamp is not None and self._timestamp >= self._INT64_MIN and
          self._timestamp <= self._INT64_MAX):
        self._normalized_timestamp = (
            decimal.Decimal(self._timestamp) /
            definitions.MICROSECONDS_PER_SECOND)
        self._normalized_timestamp -= self._WEBKIT_TO_POSIX_BASE

    return self._normalized_timestamp