To compute the number of days between two date fields, just subtract them. This requires that both dates be entered into different fields of type date.
Date2Field - Date1Field
will compute the number of days from Date 1 to Date 2.
I did have that calculation correct but I had the result field set as a date instead of a number.
Do you know of a simple way to incoporate this result feild into the PMT function to calculate payments?
The PMT function is documented in FileMaker Help. I don't really follow what you are trying to do here. The function requires a term in months. An interest rate that must be divided by 12 if the interest rate is annual instead of monthly and the principle amount.
I don't see anything there that you can manipulate to compute the values you want here.