Does Zakāt become obligatory on money saved to buy a car, which has been held for a full year (hawl), and how is Zakāt calculated on an amount that was increasing monthly, and do I owe expiation (kaffarah) for delaying the payment of Zakāt due to my ignorance of its obligation?

If saved money reaches the nisab and a hawl (lunar year) passes over it, its zakat becomes obligatory. The hawl is calculated from the time it reaches the nisab. If the money falls below the nisab during the hawl, the hawl is interrupted, and a new hawl begins when it reaches the nisab again. To calculate the zakat on a monthly increasing salary, one can refer to Fatwa no. 31961.