Countdown Timer For Online Exam

Question

Hello,
 
I am designing Online Examination System. For this I need a count down timer which does not start from beginning. Suppose timer has started from 40 minutes if the user refreshed the page at when the timer showing at 20 minutes then timer will be continue from 20 minutes. And there is also another case if the user is giving the exam and due to any problem if the user not able to continue the exam, if the user again starts the exam then the exam start from where he have left the exam and timer also starts from the remaining time left where he had left the exam.
 
Can any one suggest how can I achieve the goal.
 
Thanks,
Deepak Pandey