How can the zakat be paid on money accumulated from bank investment certificates that has not had zakat paid on it for twenty years?

It was previously explained that the ruling on investment certificates in usurious banks is that they are unlawful usury (riba), and it is impermissible to purchase them. Zakat (definition) is obligatory only on the principal value of the certificates, without the profits, because these profits are unlawful wealth that must be disposed of, based on the Almighty's saying: "And if you repent, you may have your principal - [thus] you do no wrong, nor are you wronged." The unlawful profits are to be spent on the welfare of the poor and needy. It is not permissible to deal with a usurious bank except in cases of extreme necessity, such as depositing money in a current account.