SM 9.4 - RAD - how to set a date based upon current date and a second field?

I am trying to set a date field to a date in the future based upon the current date and the number in a field. 

field: audit.days (this field would be either 30, 60, 90 or 120)

field:  (I want this field to be the current data plus how many days is specified in the audit.days field)

I know how to set the date in RAD using a static value such as tod() 60*24*30 (this works for 30 days), but I want to be able to switch out the 30 with the audit.days field, like tod() 60*24*audit.days and I cant get it to work - is there a way to do this in RAD or is there a way to do this in Javascript? I am using a button for this action.