How does one pay zakat when he does not know the date or time of zakat's obligation, and should he pay for past years, and is it necessary to know the time of the hawl's completion?

A Muslim must learn the rulings of Islamic law and the zakat (definition) on wealth for those who own it. Whoever delays zakat out of ignorance is sinful and must repent and pay the overdue zakat, because it is a debt owed by him. He must ascertain the time when his wealth reached the nisab (definition) (minimum threshold) and the amount of zakat due, so that he may pay what clears his conscience.

Sheikh Ibn Uthaymin was asked about a repentant person who had delayed zakat for five years. He replied that repentance nullifies God's right, but the right of the deserving remains, so he must pay the zakat to them. As for estimating the zakat, he should ascertain its amount to the best of his ability. For example, if he had ten thousand, its zakat is 250 per year, and he should pay it for the past years. If the wealth increased or decreased in some years, he should pay the amount for the increase or be exempt from zakat for the decrease.