Calculating interest on number of days in the period

What's the formula for calcuting the interest on a loan,
based on the number of days in the period?

I am using a couple of different "loan amortization"
spreadsheets, but these seem to assume the payment will be
made on a specific day (the first of each month for
example). For example, if the payment was due on February
1, and the last payment was made on January 1, there would
be 31 days of interest due on February 1. But, if the
payment isn't paid until February 10, there would be 41
days of interest to be calculated.

Any help you can provide is appreciated!

Thanks.