Is it obligatory to pay Zakat on money for all past years during which the money was under the father's care, given the ignorance of the exact amount of money and the annual increase?

Zakat (definition) is an obligatory duty on saved wealth. To determine when it becomes due, one should refer to the bank to ascertain when the wealth first reached the nisab (definition) (minimum threshold). If the wealth reached the nisab at a specific time, zakat becomes due at the same time in the following year. If additional money is added to the nisab, either it is combined with the original amount and zakat is paid on the total, or it has its own independent hawl (zakat year). The current nisab for banknotes is equivalent to 85 grams of gold or 595 grams of silver, and one-quarter of a tenth (2.5%) must be given out. If it is difficult to determine the exact amount each year, diligence and investigation are required to the best of one's ability.