DATESYTD not working on SWITCH with logical test based on time

[TomBI]

I am encountering a problem with a DATESYTD on a time based SWITCH measure that I am writing. 

Situation:

- I have a measure for my Actuals

- I have a measure for my Forecast

- I have a SWITCH measure that looks at actuals when the week is in the past (based on the WeekIndex_Nr), and at the forecast when the week is in the future

- I want to create a measure that calculates the YTD based on the SWITCH measure

Used measures:

Actuals = SUM( Fact_Sales[Quantity] )

Forecast = SUM( Fact_Forecast[Quantity] )

Switch ACT/FCST =

SWITCH( TRUE() ,
    SELECTEDVALUE( Dim_Date[WeekIndex_Nr] ) >= 0 , [Forecast] , [Actuals]
)

YTD on Switch ACT/FCST =

CALCULATE(
    [Switch ACT/FCST] ,
    DATESYTD(Dim_Date[Date] )
)

Problem: 

I use the DATESYTD measure to calculate the YTD value. Usually this always works, but when I apply it on the SWITCH as described above, the accumulated values are ONLY those of the Forecast, and not of the Actuals. 

Result:

Can anyone explain why this is happening and how to solve it? 

Thanks a lot! 

[amitchandak · Accepted solution]

@TomBI , Try like

CALCULATE(
Sumx(values( Dim_Date[WeekIndex_Nr] ),[Switch ACT/FCST] ) ,
DATESYTD(Dim_Date[Date] )
)

[TomBI]

@amitchandak Great! Works perfectly. Can you explain why?