The “NaN” error message means that the result of the calculation is not a number. In this case, it is not a number because you cannot directly subtract dates. You correctly figured out to use the `DATETIME_DIFF`

function to calculate the number of days between the dates. You can now combine different parts of the formula together:

The `IF`

part of the formula just checks to make sure that there are values in all of the input fields. (If any of the input fields can have a valid value of 0, this formula will need a slight adjustment.)

Notice that the main part of the formula is the same as your original formula, except for calculating the difference between the dates.

```
IF(
AND(
{UPB},
{Interest Rate},
{To Date},
{Bill of Sale Date}
),
UPB * {Interest Rate} * DATETIME_DIFF({To Date},{Bill of Sale Date},'days') / 365
)
```

You may also need to set the formatting options for the formula field to show your desired number of decimal places. The default is to round the number to the nearest integer.

If this answers your question, please mark this post as the solution. Otherwise, could you please give a bit more details and a screen capture?