Networkdays

// Something like this.
// Dates is a calendar that stores
// full days (with no time component).

|---------|=====A=====|==========|---------|==========|---------|==========|----B----|

// Legend: |====| -> non-working day, |---| -> working day

var DateTime1 = A
var Date1 = DATEVALUE( DateTime1 )
var DateTime2 = B
var Date2 = DATEVALUE( DateTime2 )
var FirstWorkingAfterOrOnA =
    CALCULATE(
        MIN( Dates[Date] ),
        Dates[Date] >= Date1,
        Dates[Day Type] = "working"
    )
var LastWorkingBeforeOrOnB =
    CALCULATE(
        MAX( Dates[Date] ),
        Dates[Date] <= Date2,
        Dates[Day Type] = "working"        
    )
var StartDate =
    MAX( DateTime1, FirstWorkingAfterOrOnA )
var EndDate =
    MIN( DateTime2, LastWorkingBeforeOrOnB )
var NonworkingDays =
    CALCULATE(
        COUNTROWS( Dates ),
        Dates[Date] >= StartDate,
        Dates[Date] <= EndDate,
        Dates[Day Type] = "non-working"
    )
var NetworkingDays =
    1 * (EndDate - StartDate) - NonworkingDays
var Result =
    IF( NetworkingDays > 0,
        NetWorkingdays
    )
return
    Result