Calculating interest on past due mortgage note

Can anyone tell me how to calculate the interest due on a note past due? The details are: Past due balloon pmt-\$2971.54
Date due–12/01/1991
interest rate 11%
It sounds crazy, I know, but it’s true…help appreciated--------

The answer depends upon the language of the note. Typically in an amortizing note, unpaid amounts due are added to the principal. If this is true for your note, then add the interest due for each payment period to the principal balance before computing the interest due for the next payment period.

For example, if the payments were to be made monthly with interest paid in arrears, then use

Balance Due = (Beginning Balance) x (1 + 0.11/12)N

where Beginning Balance = \$2971.54, and N = number of full months that have passed since 12/1/91. If you want to compute the amount due on 3/1/2008, N will be equal to 195.

If you are looking for the amount due for a mid month payoff, use the previous formula to calculate the balance due at the beginning of the month, then add the daily interest due to get a final payoff amount.

Balance Due = (Balance at beginning of month) x (1+ 0.11/365 x D)

where D = the date of the month that the mid-month payment will be made. For example, if the final payment is to be made on the 12th of the month, then D = 12