Sharedobject question?

Sharedobject question?

Is it possible to create a countdown timer that works like this scenario:

Once the user enter let say a name and submit it - the timer starts
counting down from let’s say 1 hour 30 minutes.
Now the user leave after several minutes and comes back later sometime.

The thing is, when the user return - the time spent on previous visit/s should be subtracted from the original ‘limit’ 1 hour 30 min, and continue to countdown…

How do I do that? Is using sharedobject the way to achieve this?

Thanks for reading this - if able to give me hints or help thanks in advance.

Cheers Tonni.