How is Zakat on wealth paid, knowing that the sole source of money is the monthly salary, which is entirely spent on family and dependents?

Zakat (definition) is not obligatory on the monthly salary that is spent entirely before the completion of a hawl (lunar year). However, if an employee saves a portion of his salary that reaches the nisab (definition) (minimum threshold) and a hawl passes over it, then Zakat becomes obligatory on it. The employee can track each saved amount and calculate its Zakat when a hawl passes over it, or, it is easier and better to pay Zakat on all the money he possesses when a hawl passes over the first nisab he acquired, considering any additional amounts as advanced Zakat for what has not yet completed its hawl.