¡Hola! You cannot insert custom formulas in custom columns, there is not such feature.
That means you cannot add or subtract a duration to a date.
To perform calculations you can export the view (showing the columns you need to calculate with) as Text, open the exported data in a spreadsheet app (like Excel, Numbers.) and enter your formulas there.
Read more on how to export the data here:
BTW should you need the difference between Expected Costs and Planned Costs, you may use the column 'Expected Cost Variance'
Best regards, Vicky