What is the ruling on zakat if a father regularly pays it annually on the money he possesses, but does not wait for the amount to reach the sharia-stipulated nisab, and he pays zakat on the money based on the Gregorian year, not the Hijri year, knowing that he pays the zakat in installments? And what is the method for calculating zakat for past years, which may amount to 35 years, knowing that the nisab has varied from one period to another?

This question indicates your obsessive doubts, and it cannot be answered due to its ambiguity and intertwined issues. We advise the father to directly ask people of knowledge to clarify the reality of the matter.

The nisab (minimum threshold) is a condition for the obligation of zakat. So, whoever's wealth does not reach the nisab is not obligated to pay zakat, and what they give out is considered a voluntary donation. If one possesses the nisab but some of it is consumed before the completion of the hawl (one lunar year), then there is no zakat due on them.

We advise you to stop these obsessive doubts and to seek a scholar to whom you can directly present your questions.