A complete year in the Islamic calendar should pass, starting from the very day of the nisab’s possession, without any decrease to zero during the year, even if it goes below the nisab. In case of its decrease to zero, the yearly cycle (hawl) starts from the day the nisab becomes complete again. Zakat should be given as soon as possible after it becomes due. It is possible that death may occur and thus lead to failure in fulfilling one’s obligations. If a person dies before paying Zakat, then it must be taken from his/her estate.
‘Ali reported that the Prophet (PBUH) said: “There is nothing upon you in gold, until it reaches twenty dinars. Thus, if you have twenty dinars at the end of the year, then there is half a dinar levied on it [as Zakat]. Any additional amount will be calculated in this manner. There is no Zakat on property until it has been owned for one year.” (Related by Ahmad, Abu Dawud, and al-Baihaqi).