Is zakat obligatory on money that has been held for a whole year (hawl), or on all money available when a year has passed since the nisab amount was reached?

Zakat becomes obligatory under five conditions: freedom, Islam, ownership of the minimum taxable amount (nisab), its stability, and the passing of a full lunar year (hawl). The hawl is the passage of one Hijri year over money that has reached the nisab, based on the Prophet's (peace be upon him) saying: "There is no zakat on money until a full year has passed over it." Five things are exempt from the hawl condition: produce from the land (its zakat is due upon harvest), offspring of pasturing livestock (it follows the original in its hawl), trade profits (they are subject to zakat along with the principal when a year passes on the commodity), buried treasure (khums, one-fifth, is due on it upon discovery), and minerals (their zakat is immediately due upon extracting the nisab amount).

If you own a nisab and then acquire other money of the same type that is not a profit derived from it, you have the option of either adding it to the principal and paying zakat on both together when the hawl of the original amount is completed, or starting a new hawl for the additional money, even if it is below the nisab.