Zakat Calculation: Investments

I originally asked this question which I am splitting up. Investments and shares for retirement: Pay while contributing on the contribution amount or pay when retiring and we are able to withdraw the amount? This link suggests to pay immediately, and I have also seen rulings where you are to pay when you receive it. In either case, there is the added complexity that "real value" of the investment changes daily and until we actually "realize" the investment by selling and getting cash, it is all paper gain/loss. How to deal with that?