Slight speedups using calculated values.

def _onOffset(self, dt, businesshours):
        """
        Slight speedups using calculated values.
        """
        # if self.normalize and not _is_normalized(dt):
        #     return False
        # Valid BH can be on the different BusinessDay during midnight
        # Distinguish by the time spent from previous opening time
        if self.n >= 0:
            op = self._prev_opening_time(dt)
        else:
            op = self._next_opening_time(dt)
        span = (dt - op).total_seconds()
        if span <= businesshours:
            return True
        else:
            return False