def on(i)
        seq(:on, i, :_on, :on_objects)
      end
'every month on day 2 at 10:00' => '0 10 2 * *',
'every month on day 2 and 5 at 10:00' => '0 10 2,5 * *',
'every month on days 1,15 at 10:00' => '0 10 1,15 * *',

'every week on monday 18:23' => '23 18 * * 1',

every month on the 1st