I leave most of the Sql posting to Ryan but I will post this: The Excel worksheet function Days360 in a Sql function (sans the documented bug that incorrectly calculates the days for the end of February):

CREATE FUNCTION dbo.fnDays360
(
 @startDate DateTime,
 @endDate DateTime
)
RETURNS int
AS
BEGIN
 RETURN (CASE WHEN Day(@endDate)=31 THEN 30 ELSE Day(@endDate) END) -
  (CASE WHEN Day(@startDate)=31 THEN 30 ELSE Day(@startDate) END)
  + ((DatePart(m, @endDate) + (DatePart(yyyy, @endDate) * 12))
  -(DatePart(m, @startDate) + (DatePart(yyyy, @startDate) * 12))) * 30
END