This is a per second countdown timer encapsulated in a block. All that is required is the block which can be used on any screen or block.

The timer is a java script algorithm. The date is used to get an accurate interval as the setInterval is not a reliable timer.

The algorithm could be modified to change the duration of the interval or to create a count up timer. This is left for the individual. 

The algorithm has one input parameter which allows for the countdown starting point to be passed.

The block has an event that  allows an action to be called on completion of the countdown. This can be seen in the demo application.