What spreadsheet formula would I use to automatically calculate loan interest from a past date until today?

For example, let's say I want to calculate the interest to date on a loan that was made on January 1, 2007.





How would I set up a formula so that the calculation would always be current? In other words, if I close the spreadsheet now and open it tomorow, I want it to display the correct values without me manually changing the dates.





I usually use OpenOffice.org Calc, but I also have Excel if it's necessary.





Thanks!

What spreadsheet formula would I use to automatically calculate loan interest from a past date until today?
The way I'd go about it is to load the Excel "Loan Payment Calculator" template and then just add up the interest for the period in question.