Does anyone know how to get the number of days between 2 dates, as an integer? Just doing (Date1 - Date2) gives a timespan; I need a number!

I'm trying to show the average daily sales of a stock item. I have an expression PeriodSalesQty, and a date criteria ThisPeriod. The expression DaysInPeriod=
CRITERIA.ThisPeriod_UNDERLYINGVALUE_TO - CRITERIA.ThisPeriod_UNDERLYINGVALUE_FROM +1
shows the correct number of days, but the expression
PeriodSalesQty /DaysInPeriod
gives the error "/ is not defined for objects of type System.Double and System.Timespan"

How do I convert the Timespan to a number of days?