Massallah pertaining to calculating Zakaah

If my zakaat was due on the 15th rajab and i calculated the amount due but only paid it on the 20th rajab do i
1)have to recalculate the amount as to be paid on 20th rajab or the amount stays the same for the day it became due
2)the next year do i have to still calculate my zakaat on the 15th rajab again or now the 20th rajab