Zakaat

Discharging the zakaat of previous years

Q: One has not paid Zakaat for a number of years. I'm now calculating the Zakaat for the current year. How should the liability in respect of Zakaat not paid previously be treated? Does one need to add the outstanding amount to debts and liabilities for the current year and the total amount then be deducted from one’s total Zakaat assets? If this procedure is correct, what then if, after deduction, the total amount does not reach nisaab? If the liability in respect of Zakaat not paid previously is not to be included in debts and liabilities for the current year, can the Zakaat for the current year be cleared immediately and the outstanding amount in respect of previous years be cleared gradually over a period of time?