Above Function with Two Dates

[QlikDevPune]

I have two filters on the page. COB date and CompareDate

I want to show the two rows in the pivot table when user selects cob date and CompareDate in the rows.

the first row corresponds to value - which is for Cob date

the second row corresponds to value - which is for compare date

I want to create the third column called increment which shows the difference between the values of two cob dates value.

I have used above function. 

GetSelectedCount(cobdate) = 1 AND GetSelectedCount(CompareDate) = 1 AND GetFieldSelections(LevelsIndicator)='INCREMENTS',
((Sum({<cobdate ={'$(=GetFieldSelections(cobdate))'},[metricid]={2}>}[value]))-Above (Sum({<cobdate ={'$(=GetFieldSelections(CompareDate))'},[metricid]={2}>}[value]))),

it didnt work.