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