Summarize from multiple tables

Hi Durbslaw 

please use the following 

New Hires Month Datediff =
VAR CurrentMovement =
    MAX ( 'MovementSnapshot'[AppointmentDateKey] )
RETURN
    IF (
        CurrentMovement <> BLANK ()
            && CurrentMovement <> -1,
        VAR CurrentPeriod =
            MAX ( 'Period'[YearMonth] )
        VAR MovementYear =
            VALUE ( LEFT ( CurrentMovement, 4 ) )
        VAR MovementMonth =
            VALUE ( MID ( CurrentMovement, 5, 2 ) )
        VAR PeriodYear =
            VALUE ( LEFT ( CurrentPeriod, 4 ) )
        VAR PeriodMonth =
            VALUE ( RIGHT ( CurrentPeriod, 2 ) )
        RETURN
            ( PeriodYear - MovementYear ) * 12 + PeriodMonth - MovementMonth
    )

CountNewHires =
COUNTROWS (
    FILTER (
        ADDCOLUMNS (
            SUMMARIZE ( 'MovementSnapshot', 'Period'[Yearmonth] ),
            "Value", [New Hires Month Datediff]
        ),
        [Value] = 0
            || [Value] = 1
    )
)

Durbslaw 
I think this is the time when you need to add some columns and create a relatioship between the two tables.