Calculate difference between 2 dates

anne-sophie ,

Try this code for a calculated column:

Difference = 
VAR _previousDate = CALCULATE(MAX('Table'[xDate]), FILTER('Table', 'Table'[xProject] = EARLIER('Table'[xProject]) && 'Table'[xDate] < EARLIER('Table'[xDate])))
VAR _previousValue = CALCULATE(MAX('Table'[xValue]), FILTER('Table', 'Table'[xProject] = EARLIER('Table'[xProject]) && [xDate] = _previousDate))

RETURN IF(_previousValue = BLANK(), BLANK(), 'Table'[xValue] - _previousValue)

This for measure:

_Difference = 
VAR _previousDate = CALCULATE(MAX('Table'[xDate]), FILTER(ALLEXCEPT('Table', 'Table'[xProject]), 'Table'[xDate] < SELECTEDVALUE('Table'[xDate])))
VAR _previousValue = CALCULATE(MAX('Table'[xValue]), FILTER(ALLEXCEPT('Table', 'Table'[xProject]), [xDate] = _previousDate))

RETURN IF(_previousValue = BLANK(), BLANK(), SELECTEDVALUE('Table'[xValue]) - _previousValue)

anne-sophie , Try a new column like

Column = var _1= maxx(filter('Table', [xProject] =EARLIER([xProject]) && [xDate] <EARLIER([xDate])),[xDate]) 
var _2 =maxx(filter('Table', [xProject] =EARLIER([xProject]) && [xDate] =_1 ),[xValue]) 
 return  if(ISBLANK(_1),blank() , [xValue]-_2)