How is zakat calculated and paid on monthly salary savings, knowing that the saved amount from each month's salary does not remain for a full hawl (lunar year)?

If someone saves a sum from their monthly salary that reaches the nisab (equivalent to 85 grams of gold), and a full lunar year passes over it, then zakat of one-quarter of one-tenth (2.5%) becomes obligatory on it. If the amount falls below the nisab during the year, a new year begins once it reaches completion again.