Shifting data backwards

OK, the measures you need are:

For + 3 days

+3 day = CALCULATE([Sum], FILTER(ALL(DTable), DTable[DayTable] = MAX(DTable[DayTable]) +3))

Values at + 3 or less =
CALCULATE (
    [Sum],
    FILTER (
        ALL ( DTable[DayTable] ),
        DTable[DayTable] > MAX ( DTable[DayTable] )
            && DTable[DayTable]
                <= MAX ( DTable[DayTable] ) + 3
    )
)

Aggregated total at +3 = SUMX(DTable, [Values at + 3 or less])

and if you want to display the +3days value at day level and the aggregated total at the total level:

Display at +3 = IF(ISINSCOPE(DTable[DayTable]), [+3 day], [Aggregated total at +3])

So, for +18 days, you need:

Values at + 18 or less =
CALCULATE (
    [Sum],
    FILTER (
        ALL ( DTable[DayTable] ),
        DTable[DayTable] > MAX ( DTable[DayTable] )
            && DTable[DayTable]
                <= MAX ( DTable[DayTable] ) + 18
    )
)

Aggregated total at +18 = 
SUMX(DTable, [Values at + 18 or less])

and if you want the +18 day values displayed ate Day level and the total aggragation at the total level:

+18 day = CALCULATE([Sum], FILTER(ALL(DTable), DTable[DayTable] = MAX(DTable[DayTable]) +18))

Display at +18 = 
IF(ISINSCOPE(DTable[DayTable]), [+18 day], [Aggregated total at +18])

Sample file attached