How is the Zakat calculated on money that gradually reaches the nisab, where a portion of it is spent before a full hawl (lunar year) has passed on the entire amount?

There is no zakat on saved salary unless it reaches the nisab and a full year (hawl) passes on it. The nisab is equivalent to 85 grams of gold or 595 grams of silver. Money gained after owning the nisab has its own independent hawl, and zakat is not obligatory on it until a full year passes, due to the generality of the Prophet's (peace be upon him) saying: "There is no zakat on wealth until a full year passes on it." It is permissible for one who owns the nisab to pay zakat on all their savings when the hawl of the first nisab is due, as a facilitation for them.