Check if date falls between two dates

This seems to work.

Within Two Weeks = 
VAR VendorDate =
    MAX( 'Table'[LatestContact] ) 
VAR SelectedDates =
    ALLSELECTED( 'Date'[Date] )
VAR SelectedDate = [Selected Dates]
VAR DayCount = 14
VAR DateRange =
    DATESBETWEEN(
        'Date'[Date],
        SelectedDate - DayCount,
        SelectedDate
    )
VAR WithinDateRange = VendorDate
    IN DateRange
VAR Result =
    IF(
        HASONEVALUE( 'Date'[Date] ),
        WithinDateRange,
        "Multiple Selections"
    )
RETURN
    Result

See below. You'll note the date I selected is June 1, so future dates are all showing false. You just need to replace the final logic of true/false with whatever you want to show.

My sample PBIX file is here.