def add_clock(self, timezone, color='lightgreen', show_seconds=None):
        """Add a clock to the grid.  `timezone` is a string representing
        a valid timezone.
        """
        if show_seconds is None:
            show_seconds = self.options.show_seconds

        clock = Clock(self.app, self.logger, timezone, color=color,
                      font=self.options.font, show_seconds=show_seconds)
        clock.widget.cfg_expand(0x7, 0x7)

        num_clocks = len(self.clocks)
        cols = self.settings.get('columns')
        row = num_clocks // cols
        col = num_clocks % cols
        self.clocks[timezone] = clock

        self.grid.add_widget(clock.widget, row, col, stretch=1)
Add a clock to the grid.  `timezone` is a string representing
        a valid timezone.