DATEADD: Dynamic value for the interval parameter

[dyabes]

I am trying to compare the current value of a measure against the previous period using DATEADD. 

 CALCULATE(
     [Sessions],
     DATEADD( Dates[Date], -30 , DAY )
)

I do not want the interval to be fixed at -30 rather dynamic based on the selected value of the Date filter.

var _firstdate = FIRSTDATE( Dates[Date] )
var _lastdate = LASTDATE( Dates[Date] )
var _datediff = DATEDIFF(   _lastdate, _firstdate, DAY )
return
CALCULATE(
     [Sessions],
     DATEADD( Dates[Date], _datediff , DAY ) )

Unfortunately the above DAX does not work. The _datediff returns the correct value but when applied within the CALCULATE it appears to be returning a value zero. Can someone explain why this happens and how to resolve it?

[AllisonKennedy · Accepted solution]

@dyabes You need to calculate the interval for ALLSELECTED dates, otherwise it will use the context of the visual you're calculating within. 

This measure works: 

Previous Period Sales =

VAR Int = CALCULATE(DATEDIFF(MAX(Dates[Date]), MIN(Dates[Date]), DAY), ALLSELECTED(Dates))

RETURN

CALCULATE([Total Sales], DATEADD(Dates[Date], Int, DAY))

Though be careful to name the visuals, etc well as it's confusing what it's showing when random periods are selected, might be best used with a relative date slicer only???

[dyabes]

Thank you so much @AllisonKennedy! This worked brilliantly. 

[AllisonKennedy]

You're very welcome. Glad it worked.