When figuring out the new payment for the loan, we are running into a question we can not seem to find answer to.
Here is the scenario:

We received notice from the customer about their SCRA benefits, to be effective 11/18/2016, on 12/18/2016. Their next payment date was 12/3/2016 which they paid at their original loan payment on time.

We use a calculator provided to us from our vendor. The input sections are as follows:
Loan Amount
Interest Rate
Payments per year
Number of Payments
Closing date
First Payment Date

Closing date is where we are running into an issue. We know the first payment date is 12/3/2016 but the calculator wants to automatically make the closing date and the first payment date 30 days apart. This changes the payment not by much but about a $1.

So the question is this; should we be calculating for the interest between 11/18/2016 and 12/3/2016 or should we go with the calculator and use 11/3/2016 and 12/3/2016?

The textbook answer is that your scheduled payment is based on a principal balance, a time period, say 30 days, and an interest rate not greater than 6%.

For the payment period the borrower was a servicemember, reduce the rate to 6%.

Technically you're entitled to the contractual rate from 11-3 to 11-17, and 6% from 11-18 to 12-3. Because this is a small amount you can do two calculations or just give the SM the extra two weeks at 6%.

Lets assume the balance is $1,000, the rate is now 6%, and you're using a 30/360 accrual. (Could be an actual 365, change accordingly.)

$1,000 x 6% = $60 annual interest
$60/360 = $0.166667 per diem (interest)
$0.166667 * 30 days = $5 in interest owed. (Verified by 60/12=5 when using 12, 30 day periods)

Add the $5 to the scheduled principal and you have your payment. You could add the interest difference for that 14 days, but is it worth it?

No future payment should accrue more than 6% so that means no late fees if you have the rate at 6% since late fees are "interest" for the SCRA.

Anything you do more favorable to the SM is acceptable. I would find it easy to believe you'll have more cost in time working this out than than interest you'll be owed. That's why I'd keep the math simple, round in the SMs favor and more on, especially if this is a rare occurrence.