Subtracting one date from another (e.g., DATE(TODAY())-DATE(“1/7/2019”) produces a Duration value, of the form HH:MM:SS (very similar to–but not!–a Time value). You then need to convert that into a number of days:
NUMBER(HOUR(DATE(TODAY())-DATE(“1/7/2019”)) / 24)
That should get you what you need.
Note, too, that you don’t need NUMBER() (the result will already be a number), DATE() around TODAY() (it’s already a date), or DATE() around “1/7/2019” (it’ll be recognized as a date automatically).
=SUM(History[Evaluation]) / (HOUR(TODAY() - “1/7/2019”) / 24)