Bismillaah-ir-Rahmaan-ir-Raheem.

Wa 'Alaykum-us-Salaam wa Rahmatullaahi wa Barakaatuh.

1) The passing of a year is calculated according to the Islamic calendar from the time a person owns a zakaatable amount of money (nisaab). At the end of the year he will calculate the Zakaah of the total sum of the money he possesses. Once a person's savings have reached a zakaatable amount, any money that comes in his possession thereafter will simply be added to the initial sum. It makes no difference whether the wealth increases or decreases during the course of the year, neither is it necessary that every part of the wealth remains in one’s possession for the whole year. He will thus give 2.5% of all the money he possesses at the end of the year, as Zakaah, whether or not a full year has passed on all of it. (Nurul-Iydhaah, Hidaayah, Badaa-i')

2) There is no harm in giving more Zakaah than is required of you.

And Allaah knows best.

Wassalaam.