How to add n days to date

How to add n days to date

Can I use a variable to specify how many days to add to a date?

e.g.:

Interim = 60;
Date_Performed = zoho.currentdate;
Next_Date = Date_Performed + 'InterimD';


'InterimD' doesnt work, of course and Interim can change, so I dont want to hardcode '60D'

Thanks for help!!