Net Work Days Grouped By Month

// First, you should have a Calendar
// in your model that covers all the
// years found in any of the date fields
// in any of your tables. This Calendar
// should be disconnected from the Project
// dimension.
// Second, this Calendar should have all
// the date/time entities defined like
// months, years, day names, and each 
// date should have a column indicator
// to say if it's a working day or not.
// Project dimension stores Projects,
// that is, their ID's, names and start
// and end dates.

// Then you can write this measure:

[Net Work Days] =
var __oneProjectVisible = HASONEVALUE( 'Project'[ProjectID] )
var __startDate = SELECTEDVALUE( Project[StartDate] )
var __endDate = SELECTEDVALUE( Project[EndDate] )
var __result =
	CALCULATE(
		COUNTROWS( 'Calendar' ),
		KEEPFILTERS( __startDate <= 'Calendar'[Date] ),
		KEEPFILTERS( 'Calendar'[Date] <= __endDate ),
		KEEPFILTERS( 'Calendar'[Day Type] = "Working Day" )
	)
return
	if( __oneProjectVisible, __result )

This measure returns the number of working days in the currently selected period of time for any one project that's been selected. If more than one project is visible, it'll return BLANK since you've not defined what it means "the number of working days if more than 1 project is selected." It might be that you want the number of working days that belong to any of the projects (which would be reasonable) but if that's the case, you'll need to make some changes to the code.

Best

D