Service Duration in seconds Calculation in DAX

Hi there LSHagger 

On your immediate issue of calculating the median of three values in DAX, you can use MEDIANX for this (since MEDIAN can only be used with a column reference).

For example, you can change:

MEDIAN(mod(_EndDate,1),"7:00","18:00")

to

MEDIANX ( { MOD ( _EndDate, 1 ), "7:00", "18:00" }, [Value] )

This works by constructing a single-column table containing the three values (with an automatic column name "Value") and then finding the median of that single column.

I also had a further look at the DAX expression, and you may want to consider using variables to avoid some repeated calculations and improve readability. I haven't tested the below code, but it is an initial attempt at rewriting slightly. It also avoids casting strings as date/time values:

Measure.ServiceHours =
VAR _StartDate =
    SELECTEDVALUE ( TICKET_MASTER[TICKETSUBMITDATE] )
VAR _EndDate =
    SELECTEDVALUE ( TICKET_MASTER[CLOSEDTIME] )
VAR _TimeStart =
    TIME ( 7, 0, 0 )
VAR _TimeEnd =
    TIME ( 18, 0, 0 )
VAR _DayLength = _TimeEnd - _TimeStart
VAR _NetWorkdaysStartEnd =
    NETWORKDAYS ( _StartDate, _EndDate, 1, BankHolidayDates )
VAR _NetWorkdaysStart =
    NETWORKDAYS ( _StartDate, _StartDate, 1, BankHolidayDates )
VAR _NetWorkdaysEnd =
    NETWORKDAYS ( _EndDate, _EndDate, 1, BankHolidayDates )
VAR _SecondsPerDay = 86400
RETURN
    (
        ( _NetWorkdaysBase - 1 ) * _DayLength
            + IF (
                _NetWorkdaysEnd,
                MEDIANX ( { MOD ( _EndDate, 1 ), _TimeStart, _TimeEnd }, [Value] ),
                _TimeEnd
            )
            - MEDIANX (
                { _NetWorkdaysStart * MOD ( _StartDate, 1 ), _TimeStart, _TimeEnd },
                [Value]
            )
    ) * _SecondsPerDay

Regards,