It is necessary to calculate Zakat correctly to ensure one is fulfilling this command of Allah properly. It is strongly encouraged that one reviews his accounts and finances for the previous years and the amount of Zakat disbursed. This will clarify if there is a deficit in payment. One can make amends thereafter.
If you have not been using a fixed anniversary date previously, it is highly recommended that you start doing so going forward, for more accuracy in your zakat calculations. Moreover, unnecessary delays in paying your zakat will render one sinful.
Reviewed on 30/11/2021