Index based on two date columns

This is done more easily in the query editor, but since you stated you need a DAX column, this one seems to work.  You'll need to replace Index with your actual table name.

NewIndex =
VAR vThisValue =
    INT ( CONVERT ( Index[Purchase date], DATETIME ) )
        + INT ( CONVERT ( Index[sales date], DATETIME ) ) / 100000
VAR vAdd =
    ADDCOLUMNS (
        Index,
        "cValue",
            INT ( CONVERT ( Index[Purchase date], DATETIME ) )
                + INT ( CONVERT ( Index[sales date], DATETIME ) ) / 100000
    )
RETURN
    RANKX ( vAdd, [cValue], vThisValue, ASC )

Note that I needed to use several Replace Values steps in the query editor to change month names to my locale (e.g., oktober, marts, februrary).

Pat