Rest dates

[godtooro]

Dear all!

I've two columns with dates: Invoice date and Due Date.
I want to rest and calculate the days diference between both columns but I'm obtaining the error: "more than value expected".

I'm making: 

Pending - Days between Invoice and Last Expiration Dates =

//Average number of days it takes to get a pending invoice paid / cleared

var _stardate = ALL(BIFAR000[Invoice Date])

var _enddate = ALL(BIFAR000[Due Date])

RETURN

DATEDIFF(_stardate, _enddate, DAY)

What's wrong?

[Ahmedx]

why ALL , needs to be deleted ALL
needs to be replaced with MAX