A couple of other notes that may help you.
I does not seem like wrapping DATETIME() around your DATEADD() function would be necessary. Assuming [Dt Creation Time] is already formatted as a date, [New Eval Date Calc] will just take the same data type. This may be contributing to the data type issue. Make sure that both of them are both Date or both Date/Time.
Additionally, you'll want t take a look at the link Mahfooj provided. UTC does not change for daylight savings time...without something like this, your calculation is going to be off by an hour in a couple of weeks!!