Cumulative Sum with Restricted Dates

To use ALLSELECTED you have to know how it works. Throwing functions (semi-randomly) at a formula without a deep understanding of how a function works is a sure way to fail.

Try this code:

[Cumul Sum] =
var __lastVisibleDate = MAX( CostTable[Date] )
return
	CALCULATE(
		SUM( CostTable[ColumnToSumOver] ),
		CostTable[Date] <= __lastVisibleDate
		ALL( DateTable )
	)

// Here's a different version
// that will give you the correct VISUAL
// cumulative but only if the dates
// visible are consecutive.
[Cumul Sum] =
var __lastVisibleDate = MAX( CostTable[Date] )
var __firstVisibleDate =
	CALCULATE(
		MIN( CostTable[Date] ),
		ALLSELECTED( DateTable )
	)
return
	CALCULATE(
		SUM( CostTable[ColumnToSumOver] ),
		CostTable[Date] <= __lastVisibleDate,
		__firstVisibleDate <= CostTable[Date],
		ALL( DateTable )
	)

// And this version will give you
// the correct visual cumulative 
// even for non-contiguous dates.
[Cumul Sum] =
var __lastVisibleDate = MAX( DateTable[Date] )
return
CALCULATE(
	SUM( CostTable[ColumnToSumOver] ),
	KEEPFILTERS(
		DateTable[Date] <= __lastVisibleDate
	),
	ALLSELECTED( DateTable )
)