You can use DATEADD() function to compute the 14 days before each date. Create a measure using the below code.
SUM ( 'Table'[SalesAmount] ),
ALL ( 'Table' ),
EARLIER ( 'Table'[Date] ) = DATEADD ( 'Table'[Date], -14, DAY )
While the previous DAX expression works, I'd reckon you create a calculated column which is better than the measure-approach when it comes down to performance.