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Upon checking your form, I see that you are using the form calculation widget, you can actually achieve what you want by using the default date and using conditional logic to update it. Here's what I did.
1. Replace the "Expiration Date" with a default date field. Make sure that the date format are the same, in this case its ddmmyyyy. Click "Conditions".
2. In the conditions wizard, choose "Update a form field value".
3. From here, I added a rule that when the activate date is filled the following action will initiate, here you can just add the activation date + 30 and output it on the expiration date field.
That should work. Thank you.