Comparable day in DAX

In many companies they want to compare todays sales with the comparable day last year. To find the comparable day we subtract 364 from todays date. In this sample todays date is 15 january 2007.

To see what dates we should get we can run this SQL

select '2007/01/15' as TodaysDate, DateName(dw, '2007/01/15') As 'DayName', 
dateadd(day, -364, '2007/01/15') As ComparableDay, DateName(dw, dateadd(day, -364, '2007/01/15')) As ComparableDayName

And this will return this result

Comparable day 3

To check the values for these dates we can run these DAX queries

EVALUATE
FILTER (
    ADDCOLUMNS (
	VALUES ('Date'[Date]),
        "Sales this day", 'Internet Sales'[Internet Total Sales]
    ),
    'Date'[Date] = DATE ( 2007, 01, 15 )
)

EVALUATE
FILTER (
    ADDCOLUMNS (
	VALUES ('Date'[Date]),
        "Sales this day", 'Internet Sales'[Internet Total Sales]
    ),
    'Date'[Date] = DATE ( 2006, 01, 16 )
)

The final query where we put all together looks like this

EVALUATE
FILTER (
    ADDCOLUMNS (
        VALUES ( 'Date'[Date] ),
        "Comparable day", CALCULATE (
            'Internet Sales'[Internet Total Sales],
            DATEADD ( 'Date'[Date] , -364, Day)
        ),
        "Sales this day", 'Internet Sales'[Internet Total Sales]
    ),
    'Date'[Date] = DATE ( 2007, 01, 15 )
)

Comparable day 4

Please note that this sample requires a date dimension that has the date 2006/01/16. If you are unsure about this you will have to change the query. The -364 approach will have issues if last year was a leap years.

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