Yes, @kgalbert is right on the money. Alteryx only accepts dates in the following format: yyyy-mm-dd.
You could add a DateTime tool to the start of your workflow (as long as no other tools depend on the current date format you have) to fix this, and set the "Select the format that matches the incoming string field" configuration to: MM/dd/yyyy.
You'll then have a new field called DateTime_Out (unless you rename it), which you can apply your formula to later on and avoid these coercion issues.
Cheers!
