How is Zakat to be paid on salary and fluctuating bank balances, taking into consideration how to calculate the hawl (one lunar year) for the deposited amount, and the Zakat on amounts that were not paid in previous years due to ignorance?

If the balance reaches the nisab (threshold) and a full lunar year has passed on it, zakat becomes obligatory on it. The entire wealth is to be purified (by paying zakat) when the hawl (lunar year) is completed, whether it is the original amount or what has been added to it. The date of the beginning of the hawl must be recorded if one fears forgetting it. Whoever has not paid zakat for previous years must pay it immediately, and zakat does not lapse with the passage of time, because it is a right for those who are entitled to it. Whoever was obligated to pay zakat and delayed it without a legitimate excuse has sinned, and must pay it for all the years in which he delayed. He should act according to his best judgment in estimating the wealth and the number of years if he is uncertain about them.