def _update_gmt(self):
        """
        Based on timezone and start date, the GMT card is updated
        """
        if self.simulation_start is not None:
            # NOTE: Because of daylight savings time,
            # offset result depends on time of the year
            offset_string = str(self.simulation_start.replace(tzinfo=self.tz)
                                .utcoffset().total_seconds()/3600.)
            self._update_card('GMT', offset_string)