Calculating Simple Daily Interest?

[jldugger at gmail.com]

I've got some student loans in repayment, and I'm trying to get
GNUCash to accurately model the principal interest split. According to
the DOED[1],
they use a Simple Daily interest formula, which calculates interest on
days between payments. Is there a way to represent that in a scheduled
transaction
template somehow? I scarcely understand ipmt() and ppmnt()...

Here's a schedule to give you an idea of how the split varies.
Month   Pmt        Principal    Interest

Jan'11  $81.38  $60.06  $21.32
Dec'10  $81.38  $61.33  $20.05
Nov'10  $81.38  $59.13  $22.25
Oct'10  $81.38  $60.41  $20.97
Sep'10  $81.38  $61.00  $20.38
Aug'10  $81.38  $58.07  $23.31

This has been annoying me for years, that my number for principal
remaining and the lender's number never agree. If anyone can help,
thanks a lot!

Justin

[1] http://studentaid.ed.gov/PORTALSWebApp/students/english/MonthlyInterestCalculation.jsp