I think the function does whatt the help menu says. The help on the
days360() function says the following

Returns the number of days between two dates based on a 360-day year (twelve
30-day months), which is used in some accounting calculations. Use this
function to help compute payments if your accounting system is based on
twelve 30-day months.


for the 3rd parameter = method.

FALSE or omitted U.S. (NASD) method. If the starting date is the 31st of a
month, it becomes equal to the 30th of the same month. If the ending date is
the 31st of a month and the starting date is earlier than the 30th of a
month, the ending date becomes equal to the 1st of the next month; otherwise
the ending date becomes equal to the 30th of the same month.
TRUE European method. Starting dates and ending dates that occur on the 31st
of a month become equal to the 30th of the same month.



" wrote:

In office pro 2003, excel the dates between function mis calculates i.e.
creating a amortization for simple interest using the days() between
function mis-calculates the number of days. i.e. aug 10, 2006 to sept 5, 2006
is 27 days, date() function reports 25 days, causing interest and principle
to be wrong and ending balance to be incorrect.