How is Zakat calculated for the past period when the accounts for the previous year are unknown, and should the calculation start from today or await the beginning of the next year?

This issue is known as Zakat on acquired wealth (mal mustafad). We have previously clarified that if the amount a person saves from their salary reaches the nisab, they must determine the day on which the money reached the nisab according to the Hijri calendar. Anything they save after this date does not become subject to Zakat until a hawl (full lunar year) passes over it. Its Zakat is not due with the Zakat of its principal amount. However, if the employee wishes to be lenient with themselves, they can pay Zakat on all of it along with the Zakat of the principal. They should determine the time when their money reached the nisab, and at that same time the following year, they pay Zakat on all that they possess. This is not obligatory but is more lenient for the Zakat payer and more beneficial for the poor. Zakat must be paid when a hawl has passed over the money and it has reached the nisab. The nisab is approximately equivalent to 85 grams of pure gold or approximately 595 grams of pure silver. It is not permissible to delay Zakat once it becomes due. If you are unable to determine the exact amount, then make an effort to estimate (taharri) and pay what gives you certainty or strong presumption that your obligation has been fulfilled. However, if the money has not reached the nisab or a hawl has not passed over it, then there is no Zakat due on you at that time.