How can the zakat on money for past years be calculated, knowing that the money saved for purchasing a house fluctuates, and continuous sadaqat and aid are given to family members?

If wealth reaches the nisab (minimum threshold) and a full year has passed over it, its zakat becomes obligatory. One must hasten to repent for the sin of delaying its payment. Charitable donations and aid are not counted as zakat unless they were intended as zakat and the beneficiaries are among those eligible to receive zakat.