Interest calculation in base to 30 days per month and 360 commercial year

Hi could you help to reach the days between the dates:

date1 := 30/01/2012D;


if try to make it on the navision the calcs days sum 237 days

if repeat the sum in excel with the calcs days sum 233 days this is at right result

i think navision are making the calcs between dates to 365 days and take each month as normal days and not as 30 days.