Calculate difference in time by minutes

I'm trying to automate the scoring from a baby shower game. Guests guessed the stats of the baby (length, weight, date, and time) and the winner will be whoever was closest in aggregate. I've got the other categories sorted out, but time is giving me an issue.

I'd like to have the guess cell reference the actual time cell and give a difference in minutes. My solution so far has been =ABS(Actual-Guess) this works for the other categories but spits out nonsense for time.

Is there a conversion that needs to happen or am I trying to format this incorrectly?