Date fields + Formulas

I have a date field st_Contact_Date which a user enters a date into.

I need a formula to add 30 days to that date and to populate it into the field Expiry_Date

I have followed the instructions in the deluge help guide to a tee however I keep getting the following error:

Encountered "type" at line 3, column 1. Was expecting one of: "this" ... "input" ... "TRUE" ... "FALSE" ... "groupCount" ... ... ... ... "count" ... "empty" ... "null" ... "exists" ... ... ... ... ... ... ... ... ... "(" ... ")" ... "!" ... "+" ... "-" ...

I am trying to do this as a On User Input script for the st_Contact_Date field.

Any help?