The field is duration but the formula returns a number result.

Hello,

I am a novice.  I have a field called [Application Submission Date/Time]
In my report I want to show how many days have past since the date it was submitted.  I tried to follow this example I found online but it gives me an error.

ToDays(Today()-ToDate([Application Submission Date/Time]))

I get this error:
Formula error
The field is duration but the formula returns a number result.

Am I close and just missing something?

Thank you