I borrow money for my small biz on an indefinite period., and I cant get the
answer I need bu usng Excel 2003's interest rate calculations, as this seems
to only work if there is a defintie period of time.
example:

I borrow £10 at a rate of 1% for every 14days.
Typically I repay it at anytime between 14 and 90 days.
So i want to be able to know how much interest I am clocking up on 14 day
basis.

Advice always welcome!
Thank you