How can Zakat be calculated for the past five years, given that the salary is spent from it, and multiple deposit and withdrawal operations occur on it, making it difficult to determine the amount upon which a full year has passed for each year individually?

The fundamental principle is that Zakat (definition) is not obligatory unless the wealth reaches the nisab (definition) and a full year (hawl) has passed. If someone is confused and does not know when Zakat became due on them, they must strive to estimate and pay Zakat according to their best judgment. If they can determine this from their account statement, that is better. However, if they are cautious and pay it from the beginning of the year, it is more absolving for their conscience.