Alternately, (A2-A1)*24 gives you the difference, in decimal form. (In other words, 4.5 hours).

What you're getting with A2-A1 -- that is, 0.19 (rather, 0.1875) is the number of DAYS between the two numbers. Since it's less than a day, it gives you a decimal. Multiplying by 24 hours gives you the number of hours.