Paid Subscriptions - "Stacking"?

[DarkstaR69]

So, I've noticed that paid subscriptions don't seem to stack.
I sell software and have integrated my verification into VB's subscription API, and its all good and well, for now. However, when I start bringing my payments to monthly charges instead of a 1-time-buys-all, this is going to become a problem. It has occurred to me that if user X buys subscription Y of 20 days, wait until he has ten days left and then buys it once more, his time will be reset at 20 days instead of additively becoming 30 days.

Fooling around in VB's code will make fix change easy, I'm sure, but I'm wondering if this problem has been conquered before, or if there is a simple way around it. I read VB's manuals and saw nothing, so hopefully someone here can help me before I decide to just re-code it.

Thanks in advance!
//DarkstaR~