Manually update an auto date based on @Created On

Hi there, 

I built a web enquiry app that automatically adds the date of the enquiry to a calculation field using @Created On. However we've recently started using the app to capture all our enquiries, not just the web ones and here's where the challenge lies!

I've built a great report in google data studio that allows me to filter on any aspect of the enquiry and it's working so well that I now want to add historical enquiries. 

So if I were to add a date field to the app, is there a way to amend the calculation to something along the lines of: "if there's a date in the date field, use that, otherwise use @created on?

Fingers crossed you can help me otherwise there's a lot of manual data entry on my horizon!