work out time basis for the log - new style

def find_time_base(self, gps, first_ms_stamp):
        '''work out time basis for the log - new style'''
        t = self._gpsTimeToTime(gps.Week, gps.TimeMS)
        self.set_timebase(t - gps.T*0.001)
        self.timestamp = self.timebase + first_ms_stamp*0.001