The Zakah year is based on the Hijri (lunar) calendar and not on the Gregorian (solar) calendar. Zakah should be calculated accordingly.
Q. When does a person’s Zakatable year commence?
A. The very first day that one possesses wealth equivalent to the value of Nisab heralds the commencement of his/her Zakatable year.
And Allah Knows Best
Darul Ihsan Fatwa Department