بسم الله الرحمن الرحيم
Zakah will be wajib each year on the capital and on the profits which you earn on it. Lunar year is taken into consideration in paying zakah and obligation of Zakah. Thus you should calculate your zakah as per Islamic calendar (Muharram, Safar etc.) instead of the year 2018-2020.
Allah knows Best!
Darul Uloom Deoband
This answer was collected from the official ifta website of Darul Uloom Deoband in India.