DiffDate : Not a valid expression

Hi everyone

I'm trying to create a new column that takes two Date columns from two different tables and calculates the difference.

So far, i've been trying this:

MyNewTable = DATEDIFF(FirstTable[BeginningDate], LastTable[FinishDate])

and

MyNewTable = DATEDIFF(FIRSTDATE(FirstTable[BeginningDate]), LASTDATE(LastTable[FinishDate]))

but I get an "DATEDIFF Not Recognized"

As well, how could i unclude only week days in this query?

Thanks for your help