使用日期和余额估算每月分期付款

I have a bond balance amount outstanding for different properties and the date they were registered in. I just need to add 20 years to the current date and get the months difference between the years then use that to divide the bond balance in order to get an estimate for the installment. Now a bond can only be granted if it will only take 30% of your salary so this means I must assume that the installment calculated is the 30% of the income. Now the income=100/30 * monthly installment. Now my questions if why did I need to add 20 years when I could have multiplied 20*12=240 to get the months difference for any bond given and does this rather mean I should get today's date and use that to add the 20 years then use that to cal the months difference to get the installment. I have a bond balance amount outstanding for di