The counter works well unless someone changes the system time of the computer to which counter gets changed too.
Below is the step how I reproduced this issue:-
1.)load the drupal page where the jquery coundown is placed.
2.)Once the countdown starts (lets say we have a countdown od 30 minutes) , reduce the computer time by an hour.(so If the current time is 10:00 am change it to 9:00 am).
3.)Now you will see the coundown got increased by an hour. so now the coundown value is 1:30:00.

I have reproduced this issue using above steps in jquery countown example page aswell.

I am using this module along with quiz module for automatic submission of students test and assessments. But this bug allows students to increase their quiz interval by any value they want.

I didnt find any solution to this issue. So if there's any solution , kindly let me know.

Tks in advance.
Vil