Hi,
You rather create a strategy with such functionality, as drawing on the chart (to show the countdown) is easier from strategy.
You can use the following pseudo code to update a chart object's text with the countdown text:
currentBar = history.getBar(chart.getInstrument(), chart.getSelectedPeriod(), OfferSide.BID, 0);
obj.setText(countDownTime.format(currentBar.getTime() + chart.getSelectedPeriod().getInterval() - new Date().getTime()));