The ḥawl is a lunar year of twelve Hijri months, since Allah states that the number of months with Him is twelve and that the crescents are the appointed timekeeping for mankind and for Hajj — all the timed obligations in the Sharīʿah run on the lunar calendar. Practically, you fix a Hijri date (many choose a date in Ramadan for the reminder) and assess your wealth on that day every year. If your accounts make a Gregorian year unavoidable, many contemporary scholars and zakat standards permit it on condition that you raise the rate from 2.5% to 2.577%, to compensate for the roughly eleven extra days, so the poor are not shortchanged.
Informational, not a personal fatwa. Consult a qualified scholar for rulings on your situation.