YTD or other time intelligence calculation

// You should adjust the names of
// the tables and columns to fit
// your model. These measures work
// correctly for any selection of
// periods, not only months.
 
[Closing Balance] =
var LastVisibleDate = MAX( 'Calendar'[Date] )
var Result =
    CALCULATE(
        SUM( T[Amount] ),
        'Calendar'[Date] <= LastVisibleDate,
        // This last line is not needed
        // if 'Calendar' is a date table
        // that's been marked in the model
        // as a proper date table.
        ALL( 'Calendar' )
    )
return
    Result
    
// Opening Balance

[Opening Balance] =
var FirstVisibleDate = MIN( 'Calendar'[Date] )
var DayBeforeFirstVisibleDate =
    FirstVisibleDate - 1
var Result =
    CALCULATE(
        [Closing Balance],
        'Calendar'[Date] = DayBeforeFirstVisibleDate,
        // Same remark as above applies...
        ALL( 'Calendar' )
    )
return
    Result