I is there any way to pause the time when the client is offline/not connected to network? or any button that they can pause the time? 

I want to gve every students a voucher code that has 120HRS EVERY SEMESTER to connect and used it for research purposes only

but the problem is when they input and use the voucher, the voucher will not pause even if they are not connected and it is only good for 5days.

120HRS EVERY SEMESTER because i cant gve them the whole sem...