How is Zakat on money calculated if the questioner has fallen short in paying it for over ten years, and his money has fluctuated between profit and loss in the stock market? Does he pay Zakat on the principal and profits, or on the remaining amount, and what does he owe for the past years?

Zakat (definition) is a great obligation, and neglecting it is a grave sin. Repentance is due, and whatever Zakat was missed from all past years must be paid. Zakat is not obligatory on cash unless it reaches the nisab (definition) and a full year has passed over it. The nisab for cash is equivalent to 85 grams of gold. As for shares that are traded, they are subject to Zakat as trade goods, by valuing them at the end of the year and paying one-quarter of one-tenth (2.5%). Their value at the time of valuation, not their purchase price, should be considered. It is necessary to diligently estimate the cash and the value of the shares at the end of each year for the past years and pay the Zakat.