Calculating the zakat every month after a year from receiving it.

Set a date from the first pay roll and after a year, whatever you have, give its zakat.

Assume the Nisaab level is 1500 Riyals. I had every month 2500 Riyals for first 5 months and 6th month I had 1300 Riyals and the remaining 6 months, I had 5000 Riyals every month.Do I owe Zakat at end of this year? How much I do?