Set the period for the timestamp.  If period is 0 or None, no period
		will be used.

def _set_period(self, period):
		"""
		Set the period for the timestamp.  If period is 0 or None, no period
		will be used.
		"""
		self._period = period
		if period:
			self._period_seconds = tempora.get_period_seconds(self._period)
			self._date_format = tempora.get_date_format_string(
				self._period_seconds)
		else:
			self._period_seconds = 0
			self._date_format = ''