Prior Period based on Selected Date Range

JS00 
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How it works ...

Create relationships

Create measure 

Previous start = 
DATEVALUE(
CALCULATE(MIN(Calandar[Date]), ALL(Calandar))
     -  [Previous duration]
)

Previous end = 
DATEVALUE(
CALCULATE(MIN(Calandar[Date]), ALL(Calandar))
 - 1
)

Previous duration = 
DATEDIFF(
    CALCULATE(MIN(Calandar[Date]), ALL(Calandar)),
    CALCULATE(MAX(Calandar[Date]), ALL(Calandar)),
    DAY
    )

Sales this period = 
SUM(Facts[Amount])

Sales previous day = 
var previousstart = [Previous start]
var previousend = [Previous end]
RETURN
CALCULATE(
SUM(Facts[Amount]),
ALL(datefilter),
Facts[Date] >= previousstart && Facts[Date] <= previousend
)

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Thanks for the clear description of the problem with example data. I wish everyone did that!

This solution works and does exactly what you asked.
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