Giving zakah before the wealth reaches the minimum threshold (nisaab), and mistakes in giving zakah
Question

My father used to give zakah in a wrong way, because he used to give zakah before the wealth reached the minimum threshold; he would give zakah when one full year had passed, as he says, and it was not even the Hijri year, rather it was the Gregorian year. What should he do about these years in which he used to give his zakah in a wrong way? There are years when he did not give zakah at all. What should I tell him about those years? You know that what he paid in those years is not regarded as zakah. Please note that my father is compassionate, goodhearted and kind, but this will not help him when he stands before Allah, because in fact I do not believe my father sometimes. When I confronted him about those years in which he did not give zakah, he denied it and said: I used to give it.
Praise be to Allah.Firstly:
In the answer to question no. 138703, we explained that giving zakah on wealth before it reaches the minimum threshold is regarded as voluntary charity, because the obligatory zakah does not become due on wealth until it reaches the minimum threshold and one year has passed since it reached that threshold.
Ibn Qudaamah (may Allah have mercy on him) said:
It is not permissible to hasten to give zakah before taking possession of the minimum amount of wealth, and there is no difference of scholarly opinion concerning that as far as we know. If a person takes possession of part of the minimum amount, then hastens to give zakah on it, or the zakah on the minimum threshold (nisaab), that is not valid, because it is hastening to comply with the ruling before the reason for it comes into effect.
End quote from al-Mughni (2/471).
Secondly:
The one who was heedless in giving zakah in past years has sinned, and must repent. Then if he knows how much wealth he possessed, and zakah was due on it, he must give zakah for every year that passed without him giving zakah, at the prescribed rates.
If he is confused and is not able to work out how much wealth he had, then he should try to work it out to the best of his ability and give zakah on that basis.
If he is able to work out the zakah for one year, and does not know what it was for another year, for example, then he should give zakah for the year for which the amount is known, and he should try to work it out for the year that is not known to the best of his ability, and give zakah on that basis.
See the answer to question no. 26119.
Thirdly:
It is permissible to give zakah before the time when it is due, if the individual possesses the minimum amount (nisaab). So if it will become due in Shawwaal and he gives it in Ramadan, for example, that is acceptable.
Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said:
With regard to giving zakah before it becomes due, before the reason that makes it obligatory comes into effect, that is permissible according to the majority of scholars such as Abu Haneefah, ash-Shaafa‘i and Ahmad. So it is permissible to hasten to give the zakah on livestock, gold and silver, and trade goods when one takes possession of the minimum amount (nisaab).
End quote from Majmoo‘ al-Fataawa (25/85).
See the answer to question no. 1966.
Fourthly:
zakah must be given on wealth once twelve lunar months have passed since acquiring it, because Allah, may He be exalted, says (interpretation of the meaning): “They ask you, [O Muhammad], about the new moons. Say, They are measurements of time for the people and for hajj” [al-Baqarah 2:189]. It is not permissible to delay giving it until after this time, unless that is for a legitimate excuse because of which it is not possible to give it (at that time).
Based on that, it is not permissible to use the Gregorian calendar to work out the year for the purpose of giving zakah.
The one who gave the zakah of his wealth on the basis of Gregorian months must make up the difference and also repent. He can work out that difference in a number of ways, such as the following:

You can work out the Hijri date for the first time you gave zakah, then make that the time for giving zakah in following years.
The difference between the Hijri year and the Gregorian year is approximately eleven days, so you can count the years when you gave zakah based on the Gregorian calendar, then multiply it by eleven days, then add the result to the Hijri year, and give zakah based on this calculation. We do not think that the difference between the Hijri and Gregorian dates will add up to a complete Hijri year, so that you would have to pay zakah for one year. Rather what appears to be the case is that the difference will be a number of days or a number of months at most. If you do that, then what was paid of zakah in the past will be valid, and he will only have to change the date of giving zakah every year, according to the Hijri calendar.

Fifthly:
The questioner’s words are not clear, or are not sufficiently precise. We have answered the possibilities suggested by his words.

If his father gave zakah on his wealth before the wealth reached the minimum amount for several years, then wealth that does not reach the minimum threshold is not subject to obligatory zakah at all, and what he gave is voluntary, in which case it does not matter whether he gave this wealth on the basis of the Gregorian year or the Hijri year.
If his father hastened to give zakah before it became due, but after the wealth had reached the minimum threshold, then his zakah is valid and there is no blame on him for giving it early.
If the wealth reached the minimum threshold but he delayed giving zakah according to the Gregorian year, then this was a mistake, and he must repent and seek forgiveness. If he was unaware of the ruling – which is something that is very possible, especially in most Muslim countries nowadays, as they work out calculations, payment of salaries and other financial matters according to the Gregorian calendar, then there is no sin on him at all, because confusion and mistakes are very possible in this situation, regardless of a person’s condition, age or level of education. Not being aware of these issues is very possible, so you should not be harsh towards your father or blame him too much for such things. All that is needed is to correct the mistake only.
The basic assumption is that your father is free of any debts or commitments, so it cannot be said that he owes past zakah on the basis of mere conjecture or presumption. The basic principle is that he is adhering to his religion and fulfilling his duty with regard to his zakah. So he does not have to do anything apart from that unless there is proof. As, in principle, he is to be regarded as honest, then his word is to be accepted with regard to the zakah he gave on his wealth, and it is not permissible to investigate the matter or to disbelieve him. If you think it most likely that he made a mistake or did not give any of his zakah in the past, then you have to deal with him gently until he himself gives it, or you give it from his wealth.

And Allah knows best.