What is the ruling of the Sharia and how is Zakat to be estimated for past years of unknown duration, during which wealth decreased and increased, knowing that it has not been paid for a long time?

Zakat (definition) is a pillar of Islam, and whoever withholds it is deserving of punishment in this world and the the Hereafter. Withholding it out of stinginess is a major sin. It is obligatory to hasten to pay Zakat for those who have not paid it, and to review the accounts of past years to determine the amount due for each year, which is a quarter of a tenth (2.5%) for each Hijri year. If they are unable to ascertain the exact amount, they must make an earnest effort and exert themselves to pay what absolves their responsibility, along with repenting to Allah for delaying Zakat. Repentance removes Allah's right, but the right of the Zakat recipients remains.