Calaulate interest on debt until paid

Not sure how to ask this question. I am using Adobe Acrobat X to present a form to a dealer that is used to report monthly meals tax remittance. The dealer is required to collect a local percentage of the net cost of the meal in addition to state sales tax. Each month, the amount collected must be reported and the amount collected must be remitted to the locality. I have the form created that does that. In addition, if the amount they owe (based on the calculation against gross sales * the % of local meals tax) is not remitted they pay a penalty of 10% or $10, whichever is more. I have worked out that calculation as well.

Where I am stumped is calculating the interest on the amount due if it is not reported and paid within 30 days of the due date. As an example:

Gross Sales = $1,000
Local % = 6%
Penalty = The greater of 10% or $10
Due Date = July 20, 2012
Filing Date = August 21, 2012
Gross Sales * Local % = Tax Due ($60)
PenaltyAmt = $10
Total Due = Tax Due ($60) + Penalty ($10)
Interest = 10% (.8333% per month) per year on the unpaid balance after 30 days.

Notes:

If not paid by the 20th of the month following the last day of the month being reported(January 1-31 Gross is reportable by February 20), the penalty is the greater of 10% or $10. In this scenario the dealer is late filing by 60 days therefore subject to the penalty. The interest will not be applied until the 31st day of delinquency.

The answer I am looking for in the scenario above is:

Tax Due + Penalty = $70
Interest = $.58
Total Due = Tax Due + Penalty + Interest ($70.58)

On August 21, the amount due will be:

Tax Due + Penalty = $70
Interest = $1.17 ($70 * .016666)
Total Due = $71.17

Now JavaScript can I attach to the Toal Due field in an Acrobat for to calculate the Interest and populate the Total Due.

Wow. That was tough!!